Actually usualy two different things are subsumed under the category "fascism":
fascism and national socialism / nazism
So basicaly fascism clearly needs liberalism as prereq, as it is clearly an anti-liberal/individuell movement. Does it need communism as a prereq? Well, that depends on the precise kind of facism. German fascism / nazism does indeed need communism as a prereq. The italian idea of fascism not necessarily so.
Exactly. Which is why Communism is an "Or" PreReqTech, not an "And" PreReqTech.
If you look at the TechTree, you'll see Fascism's PreReqTechs are the following:
Requires: (Communism or Corporation) AND Nationalism.
The Corporation aspect is a reflection of Mussolini's "version" of Fascism, which actaully existed before Hitler's Nazi party. I'm not going to explain Mussolini's Fascist theories here, instead if you're intersted... Wikipedia has a great section on the History of Fascism.