communism and facism are not diametrically opposed

They are diametrically opposed, though. Communism is a primarily internationalist movement which sees the state as an instrument of class domination and seeks ultimately to dissolve it. Fascism is a nationalist movement which glorifies an eternal state and argues that human values and liberties have no essence outside of it. These differences can't be reconciled.

Here's a wiki article on Red Facism, which identifies Stalin as a primary example.

There is a rich history of people calling their political enemies fascists. The article on Stalin's own pet theory of social fascism is even longer and better researched than that one, and the subject has an extensive history of academic discussion. But just like red fascism, social fascism was created and popularized by people with an obvious political agenda they were trying to push, and although we can all see how there are commonalities in both directions it feels clear to me that neither theory has any historical or etymological grounding. But they both make for pithy insults -- everybody knows what you're saying when you call Stalin a red fascist -- and I'm OK with that.

I stand by my position that Stalin is a facist, since (even by your own definiton) he fits the bill.

What definition? You mean "palingenetic ultranationalism"? In what way did Stalin glorify or seek the return of a previous state of Russian affairs? Mussolini had his "Empire" (even the name "fascism" harkens back to Rome), Hitler had his reincarnated "Reich", and Stalin had... what?

Regarding nationalism, Stalin strongly opposed it in his rhetoric, claiming (like Lenin did, and communists today still do) that it was contrary to class solidarity... but in practice he cultivated a certain patriotic "identitarianism" that could easily be described as nationalism... so I'll give it that.

Also I just realized this post is 15 hours old. Why am I here :/