Welcome to AA!
1. For awhile.
2. Yes, but not a common pleco for the longterm. They get 18"+.
3. Yes, but not for the longterm. They get large!
I'd recommend choosing between a betta ("fighter") or guppies. They do not coexist well at all.
RTS are cool fish, but they do tend to get nippy and territorial as they mature. Rainbow sharks look very similar, but are a bit more chill IME.