Smash 4 Custom Singles Rule Set:
All matches must be played using the following rule set:
General:
Two (2) stock.
Six (6) minute time limit.
All items off.
Customizations on.
Equipment is NOT legal.
Mii Fighters are legal.
All matches are best two (2) out of three (3).
If time runs out during a match, Sudden Death will never be played. Use the following rules instead:
If one player has more stock remaining than the other, the victor will be player with the most stocks.
If the stock count is tied, then the player with the lowest damage percent wins the round.
If Sudden Death is forced by one player using a “suicide” move (Kirby/Dedede Swallow, Ganondorf Flame Choke, Bowser Claw, Wario Bite, etc.), then whoever initiated the suicide move wins the game.
If both the percent and stock are tied at the end of a match, a one (1) stock, two (2) minute match will be played. This match must be played on the same level as the game prior and with the same characters. Whoever wins this 1-stock playoff wins the game.
Any action that can prevent the game from continuing (i.e., freezing, disappearing characters, game reset, etc.) will result in a forfeit of that match for the player that initiated the action. You are responsible for knowing your own character, and must be wary about accidentally triggering one of these effects.
Stalling is banned. Stalling: the act of deliberately avoiding any and all conflict so that one may make the game unplayable.
Olimar’s Custom Down B 2 (Order Tackle) is banned.
In the event that another incredibly unbalanced custom is discovered, this ruleset may be adjusted accordingly.

The first match of a game must be played on a Neutral Stage:
Battlefield
Final Destination
Lylat Cruise
Smashville
Town and City

Stage striking will be used to select the first stage. The lower ranked player will strike first, choosing a stage from the neutrals list to remove from consideration for the first round. The higher ranked player then chooses which stage to play on.
The loser of the first game will then ask the winner to ban two stages. (NOTE: banning Omega Versions of stages also bans Final Destination.) They must do so, or their counterpick is invalidated. The loser may then choose a stage for the second round. This process is repeated for game 3 if necessary.
When choosing a stage, you may pick any Neutral stage, or any of the counter pick stages listed:
Duck Hunt
Skyloft
Halberd
Castle Siege
Delfino Plaza
Donkey Kong 64
Omega Version of any stage

Any map may be played as long as BOTH players agree to it.
You may NOT pick a stage that you have already won on for your counterpick unless your opponent agrees.
After the stage is decided on, the winner announces their character and what custom set they will be running in a numerical format. The loser then announces their character and custom set they will be running.
If the winning player fails to announce their change in character exit out of that game immediately.
The game will still be played on the same stage.
The winner from the previous is now locked into their character.
The loser from the previous match may now reselect their character.

Customs infographic for reference.
http://i.imgur.com/GscIQpB.png
http://i.imgur.com/EmpTBf6.png
http://i.imgur.com/esks76s.png