The leaders in Civ are usually supposed to represent some of a civilization's most sucessful leaders, not their most influential. Yeah, there are a few exceptions like Montezuma, but this is the overall trend.
Frederick the Great was an extremely important Prussian leader who led the country during what was probably its most important war, and Germany would not exist as a nation without Bismarck. There is no way you can make a case for Hitler being more important or sucessful than either of those two.
Stalin (not sure why you have included him as a fascist) was also a poor leader, he miscalculated by making a pact with Hitler and not preparing for the inevitable betrayal, and his economic policies led to widespread famine. I can see someone arguing for better leaders than Catherine or Peter, but there are many who deserve to be there before Stalin.
Churchill . . . I don't understand why you grouped him with the fascists. I've got no arguement against why he shouldn't be in the game, other than the fact that there are a large number of great English leaders who he had to compete with.
As for Mussolini, the Romans do not equal the Italians. The modern Italian nation is in no way the same ethnic people as the ancient Romans, they are in fact a mix of the various barbarian groups which overran the Western Roman Empire. Neither are they politically or culturally related to the Romans at all. Mussolini therefore could never be in the game as his civ is not. If you insist on labeling the Romans as Italians, than there are hundreds of better leaders than Mussolini, including but not limited to: Scipio Africanus, Pompey, Augustus, Claudius, Vespasian, Hadrian, Marcus Aurelius, Constantine, Justinian, and Julian.
In short, even if it were completely PC to put the Axis leaders in the game, there would be no reason to whatsoever, as they were terrible leaders.