There is [email protected] but generally you wanna stick to the bigger places like [email protected] due to the simple fact that the usercounts just aren't that big, and that gets worse the more you subdivide a given fandom.
I do try to cross-post to smaller communities, nothing stops you from posting to every suitable community. That way, the smaller places have a way to grow, even as they semi-hibernate in terms of activity most of the time.
Yeah the crosspost feature is important if you want it to show up in multiple communities. That allows clients to filter out the duplicates if the user wants it that way.
Just ignore what this nerd said (ily MentalEdge). Its like cross posting on reddit. You press the cross post button, select the community you want to crosspost to and hit send. Just read the rules but that is also nothing new.
Suggestion: Post to the small community, crosspost to the big. I always check the original post and that's taken me to some niche communities. I'm guessing I'm not the only one who does that.