You could go with one clown, but be careful what kind..... some can get quite large. A tank-raised Ocellaris would probably be your best bet.
Tank is way too small for a Snowflake, and there aren't any puffers that I know of that will work in a 20.
I'd wait for any kind of starfish..... they need well established systems in order to have enough to eat.
Wanna calculate the cost of that new reef tank?? Take your worst nightmare, double it, and you're halfway there.