For a 20g tank there are lots of options! have you checked out livequaria? They have a whole section of nano fish to chose from.
Possum wrasse is a great idea too! I like clown gobies as well, I have a citron goby which is a very large clown goby pretty much. Also kinda unusial to see.
I know some other people have nixed your idea of the lawnmower blenny, but to me it might be doable. They are mostly perchers so they dont really require a whole heck of alot of space. I am also biased though, all my tanks have a lawnmower, just love those guys. But you'd have to stock accordingly as the lawnmower can put out a good amount of poop. lol