e.g. I’d like to make a post about a very niche hobby, but they are split in even smaller communities (many with same name in different servers).

Do you just post in the bigger one? Is it common to cross post?

Yes, usually post in the largest one - we have a lot of people that try to fold together communities into one by closing inactive communities and redirecting. Piefed automatically folds together linkposts which is the equivalent of crossposts. There's no crosspost function akin to Reddit though, if that's what you mean.