You are mixing terms (like many btw so it's nothing personal) and doing so is an obstacle to getting to the truth of the matter - if that is what you are after like me. Neither Marx nor Stalin nor even Hitler were fascists. Marx died before fascism was even conceived. Fascism is a strictly Italian phenomenon (see Marinettis' manifestos 1909, 1919). This term was never used by the German National Socialists (Nazis). It was a concrete political ideology: ultra-modern, radically anti-conservative and even substantially pro-Jewish until close to the outbreak WW2. Many Jews took part in the 1922 March on Rome, took part in the fascist government and party, e.g. Ettore Ovazza, who even ran a fascist Jewish paper "La nostra bandiera". Fascism had been started to be used as a stigmatising propaganda term in the 1920s first by Soviet marxists and by the Western Marxists, e.g. founder of freudomarxism Wilhelm Reich in the 1920s - in three separate meanings.

It was used against socialdemocratic politicians who were called social fascists by the Komintern (quotes from Zinoviev at 1922 Fourth Congress) because they opposed armed revolution and instead wanted to improve the life of the working class - which is a reactionary act from classical Marxist standpoint.

Reich used the term fascist as everyone who opposed his sexual revolution ideas - which btw included Soviet communists after Lenin's death and even German and French communists who opposed the demoralization without any barriers which he pushed for.

After WW2 fascism was used as a replacement to what hitlerism really was, i.e. "national socialism". NSDAP was a socialist party, as its name says and this is a cumbersome fact to socialists and the whole left, so they prefer to use the misleading term "fascism".

As for Marx, when you analyze closely, he in fact had no original ideas besides a single one - immediate post-revolutionary terror to keep utopia going - and he was very stubbornly persistent about it till his death and for very practical reason. He broke the First International and Gotha congress for that reason. All other elements of his doctrine are plagiarized from other thinkers and only assembled together like a applied political ideology engineer. Lenin and Stalin were his best followers - they understood this very essence of his theory - terror, and they brought it to mastery levels. Stalin - for very imperial and self-serving dictatorial reasons at that. What happened in Soviet Union was precise implementation of Marx'es ideas - though the revolution itself was more opportunistic, i.e. blanquist and realized for geopolital reasons and arranged and paid by in secret by Germany to force Russia out of WW1 and into disaster - and it worked.