ERAMEGATEN: EVERYTHING TO KNOW ABOUT FUSION:
-All Demons x Demons of the same Race will give you an Element-Race Demon
-Fusing an Element-Race Demon with another Demon will "Rank-up" or "Rank-down" the non-Element Demon
-Elements can't fuse with each other (so no Mitamas)
-Hero-Race demons (the result of Zoma x Demon) cannot fuse with anything except themselves
-Machine-Race demons cannot fuse with anything, and cannot be obtained as fusion results (except for Ruukoto with the PC-98 2hu plug-in)
-Rumor-Race demons, Hero-Race demons, Kyoujin-Race demons, Meta-Race demons, and Machine-Race demons appear in the compedium but cannot be Summoned from it. You'll have to get the demons manually (or just use debug)
-Another 2hu thing (I don't think this mod mixes well with the Fusion System): When fusing an Element-Race Demon with a 2hu, the rank-up will result in a NORMAL SMT Demon of the same race, not another 2hu.
-Some Demons have Races that don't appear here, This means that those Races are actually renamed. The list of renames is this:
+Holy, Avatar, Wilder and Vermin (for some reason) are actually Beast
+Night and Genma are Yoma
+Undead, Haunt, Jaki, Spirit and Ghost are ALL Foul
+Reaper is Tyrant
+Femme is Brute (Even if it sounds like a Lady-Race rename); Tsukumo (Unique Race of the Tsukumogami sisters from 2hu 14) is also it
+Mermaid (Wakasagihime-exclusive) and Celestial (Tenshi/Iku-exclusive) are Megami
+Drake and Dragon are Snake
+Funnily enough, Divine and Fallen are considered DIFFERENT Races by the compedium, even if Fallen-Race Demons can only be obtained with the Goetia Software
+Kishin is a male version of Lady and Deity is a male version of Megami, therefore they have basically the same fusion results
+Jirae is Fairy
-Rumor-Race Demons aren't avaliable as Fusion results. If you see something that gives You a Rumor, it's actually a Foul (funnily enough Rumor is the Race with the most Demons)
-All children you have with Demons CAN be fused, but they only fuse with themselves and the result is always an Element-race Demon
-"Fusion with You" fuses any married Demon with the Player, giving them access to MP if they're a Demon Summoner and giving them the fused Demon's skills. This is however really useless since the Player can already learn Auto skills through Skill Cards and Custom Skill Set-up is a thing that exists.
-A couple Demons need to be beaten first in order to unlock them for fusion, usually Event route-end bosses and certain Colosseum fights.

NORMAL FUSION TABLE:
Fairy:
x Foul: Beast
x Flight: Divine/Fallen
x Lady: Yoma
x Yoma: Brute
x Beast: Megami
x Brute: Holy
x Rumor: Brute
x Snake: Brute
x Megami: Brute
x Divine: Megami
x Fallen: Megami
x Tyrant: Snake
Flight:
x Foul: Yoma
x Lady: Beast
x Yoma: Foul
x Beast: Snake
x Brute: Yoma
x Rumor: Foul
x Snake: Beast
x Megami: Fairy
x Divine: Megami
x Fallen: Megami
x Tyrant: Divine/Fallen
Foul:
x Lady: Tyrant
x Yoma: Fairy
x Beast: Snake
x Brute: Flight
x Rumor: Lady
x Snake: Megami
x Divine: Brute
x Fallen: Brute
x Tyrant: Lady
Lady:
x Yoma: Tyrant
x Beast: Yoma
x Brute: Foul
x Rumor: Megami
x Snake: Beast
x Megami: Divine/Fallen
x Divine: Megami
x Fallen: Megami
x Tyrant: Foul
Yoma:
x Beast: Flight
x Brute: Fairy
x Rumor: Fairy
x Snake: Tyrant
x Megami: Divine/Fallen
x Divine: Lady
x Fallen: Lady
x Tyrant: Brute
Beast:
x Brute: Tyrant
x Rumor: Flight
x Snake: Flight
x Megami: Fairy
x Fallen: Flight
x Divine: Flight
x Tyrant: Yoma
Brute:
x Rumor: Snake
x Snake: Foul
x Megami: Snake
x Divine: Foul
x Fallen: Foul
x Tyrant: Fairy
Rumor:
x Snake: Beast
x Megami: Tyrant
x Divine: Yoma
x Fallen: Yoma
x Tyrant: D/F
Snake:
x Megami: Flight
x Divine: Yoma
x Fallen: Yoma
x Tyrant: Beast
Megami:
x Divine: Lady
x Fallen: Lady
x Tyrant: Lady
Divine/Fallen:
x Tyrant: Snake

ZOMAS:
Zomas are special Demons, created by fusing a Dolly Kadmon (purchased at the Macca Exchange for 500 Macca) with any Demon. The Zoma's gender will be the gender of the sacrificed Demon. Zoma fusions that use special software are as follows:
-Fairy:
Yuma Chitose = With Incubator Software. She's mainly a healer but starts with more STR than INT/MAG. And only gets two STR skills.
Gets Amrita, Samarecarm and "I'll Heal You!" wich are all great skills, and you can get her relatively early.

-Foul:
Gertrud = Incubator Software + Grief Seed. Is apparently a Phys attacker + Ailments. Does not have a meguca counterpart.
Kohaku = Kaleido.

-Lady:
Homura Akemi = With Incubater Softawre. No resistances, only weaknesses, and one of them is almighty. mixed attacker, gets both Gun and Mind skills, and her final Skill, Black Corroding Wings, is a MAG skill wich reduces DEF when it hits. Also gets Bites the Dust, which can lead to shenanigans if the enemies are weak to Nerve.
Glasses Homura = With Incubator Software, the Lady-Race Demon needs to be Kikuri-Hime or Kyouko (the 2hu one, not the meguca). Starts
with worse stats and her moveset is centered around acting like a human character. NOTE THAT THE ZOMA ALSO HAS TO BE LOW-LEVEL
Homulilly = Incubator software + Grief Seed Equipped. This is the PSP Homulilly, not the Rebellion Story one (althought this one is used in place of the rebellion one in the False Mitakihara Dungeon). Basically magical-oriented Homura.
Akiha Tohno = Kaleidoscope. Clearing the third Melty Blood event appears to unlock something related to her.

-Yoma:
Mami Tomoe = Incubator Software. Gun skills, gets a passive that makes her Gun skills bypass resistances.
Candeloro = Incubator Software + Grief Seed.
Sion Eltnam Atlasia = Kaleido.

-Beast:
Kirika Kure = Incubator Software. Havoc-focused attacker with eva/acc buffs and debuffs. Comes with Auto-Sukukaja and a skill that hits twice and inflicts Sukunda on the opponent.
Charlotte = Incubator Software + Grief Seed. Really fucking strong, gets access to Power Charge and strong Strike skills.
Dubhe = Dubhe needs to have been beaten first. You need a high-level Zoma for this fusion (I was able to get it by making a Zoma out of Doppelganger and then fusing that with a level 30+ Beast).

-Brute:
Kyoko Sakura Incubator software. Focused on Strike, high speed, and evasion.
Ophelia = Incubator Software + Grief Seed. Kyoko but with Agi.

-Rumor:
Satsuki Yumizuka = Kaleido. The second Melty Blood event must be cleared first.
-Snake:
Sayaka Miki = Incubator software. Physical attacker with heals and life regen.
Oktavia Von Seckendorff = Incubator + Grief Seed.

-Megami:
Madoka Kaname = Incubator software, is both a physical attacker and a magic user. Gets Pierce skills, including a Pierce that deals damage based on MAG and has a chance to insta-kill. Also gets Dekunda.

-Divine/Fallen:
Ciel = Kaleido.
Nagisa = Incubator software, seems to be ailment-based.

-Tyrant:
Oriko Mikuni = Incubator software, strong magical attacker. Gets access to Concentrate and a skill that's double Sukukaja (but lowers Atk and Mag)

-Element:
Hisui = Kaleido.

-TRIPLE FUSION TABLE (WIP):
First of all, a few things to remember:
-Disregard EVERYTHING regarding double fusion
-For Elements, It's not just demon of the same race x 3
-Zomas are unable to fuse into anything (That I can see)
-Divine and Fallen have DIFFERENT results, but if they are the result of a fusion, the result is keeping the Divine/Fallen dynamic (Result is Divine unless the Goetia Software is used)
Fairy:
Triple Fairy: Element
x Fairy x Snake: Brute
x Fairy x Megami: Beast
x Fairy x Flight: Divine/Fallen
x Fairy x Brute: Beast
x Fairy x Yoma: Beast
x Fairy x Foul: Beast
x Fairy x Divine: Brute
x Fairy x Fallen: Megami
x Fairy x Beast: Megami
x Fairy x Tyrant: Brute
x Fairy x Lady: Brute
x Fairy x Rumor: Brute
x Snake x Megami: Foul
x Snake x Flight: Yoma
x Snake x Foul: Flight
x Snake x Brute: Flight
x Snake x Lady: Tyrant
x Snake x Beast: Tyrant
x Snake x Divine: Flight
x Snake x Fallen: Foul
x Snake x Snake: Element
x Snake x Tyrant: Element
x Snake x Rumor: Snake
x Megami x Flight: Yoma
x Megami x Yoma: Megami
x Megami x Brute: Element
x Megami x Lady: Megami
x Megami x Beast: Tyrant
x Megami x Divine: Foul
x Megami x Fallen: Yoma
x Megami x Rumor: Snake
x Megami x Megami: Brute
x Flight x Yoma: Yoma
x Flight x Foul: Brute
x Flight x Brute: Snake
x Flight x Lady: Fouk
x Flight x Beast: Fairy
x Flight x Divine: Fairy
x Flight x Fallen: Element
x Flight x Tyrant: Beast
x Flight x Flight: Megami
x Flight x Rumor: Yoma
x Yoma x Foul: Flight
x Yoma x Brute: Element
x Yoma x Yoma: Brute
x Yoma x Lady: Snake
x Yoma x Beast: Tyrant
x Yoma x Divine: Yoma
x Yoma x Fallen: Foul
x Yoma x Tyrant: Beast
x Yoma x Rumor: Snake
x Foul x Foul: Snake
x Foul x Lady: Fairy
x Foul x Beast: Divine/Fallen
x Foul x Divine: Divine/Fallen
x Foul x Fallen: Flight
x Foul x Rumor: Flight
x Lady x Lady: Yoma
x Lady x Beast: Flight
x Lady x Divine: Brute
x Lady x Fallen: Lady
x Lady x Tyrant: Beast
x Lady x Rumor: Fairy
x Beast x Beast: Fairy
x Beast x Divine: Fairy
x Beast x Fallen: Lady
x Beast x Tyrant: Flight
x Beast x Rumor: Tyrant
x Divine x Divine: Megami
x Divine x Fallen: Lady
x Divine x Tyrant: Brute
x Divine x Rumor: Tyrant
x Fallen x Fallen: Lady
x Fallen x Tyrant: Yoma
x Fallen x Rumor: Tyrant
x Tyrant x Tyrant: Snake
x Tyrant x Rumor: Beast
x Rumor x Rumor: Snake

UNIQUE FUSIONS/MUTATIONS:
-Any 2hu from the Sixth (first in Windows) game x Zoma: Sakuya
-Any Magician or Nitori or Reimu x Zoma: Marisa
-Any Buddhist x Zoma: Byakuren
-Any Taoist x Zoma: Miko
-Suika, Yukari or Marisa x Zoma: Reimu
-Any one of the MoF Gods x Zoma: Sanae
-Joon x Zoma: Shion
-Kasen or Shinmyomaru x Zoma: Sumireko
-Junko x Clownpice: Hecatia
-The Tsukumo sisters fused with each other: Raiko
-Madoka x Homura: Ultimate Madoka (only Unlocked if you sided with the Incubators in the final event of the Madoka Magica questline)
-Madoka x Homura (Grief Seed Equiped): Krimheild Gretchen (Only unlocked if you sided with Homura in the final event of the Madoka questline)
NOTE: Siding with Homura also gives you Madoka's Ribbon, which I believe can be used to create Ultimate Madoka as a Persona.
-Ultimate Madoka x Homulilly: Demon Homura (need to NG+ for this one or just cheat with debug)
-Lilim x Nekomata (Kaleido activated): Len
-Lilim x Yuki Jyorou (Kaleido activated): White Len
-Shiki x Doppelganger: Shiki Nanaya (Requires the Software that allows cross-gender fusion)
-Sion x Archangel: Riesbyfe (Requires the Software that allows cross-gender fusion)
-Sion x Chimera: NRVQSR Chaos (Requires the Software that allows cross-gender fusion)
-Sion x Vampire: Warachia (Requires the Software that allows cross-gender fusion)
-Asura x Valkyrie: Asherah
-Jack Frost x Pyro Jack x Incubus: Black Frost
-Lachesis x Atropos x Clotho: Norn
-Rangda x Barong: Shiva
-Pixie = High Pixie
-Lilim = Shoakuma Lilim (Based on the Lilim design from Soul Hackers/Strange Journey)
-Cirno = Advent Cirno
-Glasses Homura = Homura
-Chefei has an evolution line based on all its forms from SMT If...
-Angel goes through all the angel tiers (Archangel-Principality-Power-Virtue-Dominion-Throne-Cherub-Seraph) (Gender changes at it evolves tho)
-Pixie (Book of Raziel activated): Sylph
-Cool -> Death Cerberus (Cool ver.)-> Hell Cerberus (Cool ver.) -> King Cerberus (Cool ver.)
-Veil -> Death Griffon (Veil ver.)-> Hell Griffon (Veil ver.) -> King Griffon (Veil ver.)
-Calica = Rangda