One core idea of fascism is that in unity lies strength. The main symbol of this is the faces, where the rods are made stronger by being made together. This idea is not incompatible with socialism, on the contrary. Now of course there's the flip side, which excludes everyone who are not in the group. Stuff like Jews are not true Germans or something. That side I would say is indeed incompatible with socialism.

But try for a moment to see it from a fascist's perspective. Who they care about, really? It's easy to only see one's peers, and not so hard to feel contempt towards outsiders. I bet that from the inside, fascism can easily feel socialist. A current example I think would be some anti-fascist groups, which despite their stated goals can be pretty fascist.