Autism

I'm sorry but I completely disagree, read the room is entirely a rule. If you think social expectations are merely skills and not rules then idk where to really take this because society, socializing, it's all rules of which skill can allow you to bend and sometimes break. For instance it's against the rules to be happy at a funeral, even if you're happy but if you're socially skilled you can manage it. Or how it's against the rules to answer honestly if asked how you are doing. For me, small talk is excruciating, I hate doing it but I can't tell people I don't care to do it or just stop because it's against the unspoken rules.

I think you're getting stuck on people saying "read the room" not all the unspoken rules that ND people have to navigate simply because not doing it is rude. If I get asked "how are you" and I reply "I don't know why I'm alive anymore" I am considered an asshole not the person asking questions they don't want answers to. I have to follow the unspoken rules that they don't really give a fuck about me, they don't care how I'm doing, and that I need to lie even if I'm uncomfortable with it because they forced me to.

Do autistic people, as individuals, not have rules in their own head about how people should interact with them?

Autistic people generally have either far fewer than allistics or if they have some kind of social obsession potentially they have a whole world of rules of their own that even allistics will struggle with.

But yeah generally, in my own case: 1) Don't be irrationally or sadistically mean.

That's basically it. You can be irrational/strange around me and at most I'll be surprised due it being unexpected and my "mask software" wont have a response to load and I'll freeze up for a bit. You can even be mean if there is sufficient justification for it. Maybe I fucked up bad.

Now, "being mean" a fairly broad category and I have specific obsessive silos of topics I don't want broken, but that's on the basis of a "info hazard". Mainly: discussions of poop or story spoilers. But if someone ends up breaking those "Rules" I don't hold it against them because they could not have known that I have a severe aversion to both of those things, I just warn them and move on.

In social groups composed entirely of autistic people, would another set of norms emerge that could get someone in the group scolded if they broke them, just like in the rest of society?

Yes but they'd probably be documented, FAQ'd, etc. Autistic people would tell rule violators to RTFM.

When a NT person upsets an autistic person because they broke a norm they weren’t familiar with, wouldn’t they also feel bad and try to remember not to do that in the future?

If the autistic person got upset at the person for breaking a norm the allistic was unfamiliar with they'd be being unfair assuming there was no good reason for them to have known in the first place.

Some taboos exist for good reason and apply across the board. We don’t greet strangers by asking them how their genitals are feeling, for example (although that would be hilarious).

I unironically would be pretty comfortable in a society that did that. At worst I'd probably be confused by why this was the thing people asked about but if I encountered a society that did such and I learned that as a common greeting I'd settle in fine.