So long as the 'public vs. private' dichotomy exists, I believe public outlets should be subject to strict regulations that force them to put out factual and non-biased content. This would essentially mean censoring all ideological speech on public platforms unless it was put forward in an 'overview' kind of way. Public stations in my country, for example, will hold interviews with all kinds of people, and they have the freedom to explain their positions. As long as they don't use it as a propaganda outlet this is fine.

Private outlets matter less. The rules regarding a private entities right to set their own rules regarding speech are hardcoded and won't change as long as they're around. You can't force Wal-Mart to uphold any specific standards regarding these policies, it's regulated internally. I don't care about this. Attempting to change this would be a waste of time.

In a socialist society, I don't see there being one definite rule as to how speech is handled. If some asshole literally builds a shack on the side of the road to hand out copies of Mein Kamf that he printed himself, communities should be able to vote that his shack be taken down. If he doesn't comply, that's illegal, and it's subject to whatever the terms of the legal system are. That's not an infringement on free speech in my opinion, it's about the freedom to associate (or not associate) with whoever you like.

Setting a 'no fascism, ever' rule across the board seems arbitrary to me. If that's the way society feels then it's all good, and they'd handle it as I explained above, or in whatever way they'd like. If that's not the way society feels, it won't do much to deter the spread of fascism. Socialism itself has thrived underground and through underground publishing in history and so can fascism.