some good fish would be ocellaris or perc clowns, royal gramma, pseudochromis (i like the orchid variety myself, less agressive) maybe a citron goby, chromis... those are all pretty much peaceful fish that will not get too large.
were you thinking about going with a reef tank? if so, plan now with what fish you decide to add- make sure they are reef safe!
i had a 20g at one point with a lawnmower blenny, an occellaris, and two ytail blue damsels before i upgraded, it was ok when they were small but it got a little crowded when the damsels decided they really didnt like eachother.