I have a 12 gallon with a Tailspot Blenny, Clown Goby, and Wheeler's Shrimp Goby. I like them all, but the clown goby might be my favorite.
I think a Royal Gramma would work in a tank that size. I've heard dottybacks are monsters, even more so than clowns, but I've never actually kept one. Clowns can be hit or miss. Some people keep them in that sized tank with no problems, but others have problems.
You should look at those wrasses, they're quite cool. I considered a Possum wrasse for my tank, but they slightly too expensive and IMO too large. Online sources say that they get ~3 inches, but I swear the one I saw at my LFS was closer to 4 inches.
A Geometric Pygmy Hawk could work too. I don't know that much about their behavior except that they're small enough to fit in the tank you have.