That's a good question about the clown and the answer could dictate future additions.
In general, I'd say heck yeah, you can add fish. It's not just number of fish. It's size, behavior, territory, food quantity needed, etc., etc. You'd be better served asking if you can add a particular fish that you're considering than just asking if you can add fish.
Good luck.:bounce1::bounce2::bounce3::lolspin::bounce3::bounce2::bounce1: