I would say that the gulf between Communism and Fascism was actually a lot smaller than that between Social Democracy and Fascism. Make of that what you will.
The gulf between communism and fascism is very small indeed. Hitler wanted communists to join the Nazi Party because communism was more appealing than fascism.
I didn't say very small.
Qualitatively, I'd say it goes something like Nazism - Conservatism - Communism - Liberalism - Social Democracy (all terms in European context of course).
But I've always been of the opinion that National Socialism was rather distinctly different from all the other ideologies and isn't really a natural bedmate of any of them.
(for instance I think Hitler wanted everyone to join the Nazi party, except for the üntermensch. That is partly why they were so successful - at least rhetorically they weren't stuck in the bitter class divide of traditional politics. And before someone asks I'm making all of this up, so it might well be wrong)