The Dr.'s have a page of all nano fish.
I have had a clown goby (green) and i thought it was a little fish with a lot of personality. Likes to perch on a few of his favorite rocks or corals and just people watch.
Have also had the wheeler's goby and he was a nice fish as well. Once they get comfortable they will be out looking for food quite often. Takes them a while to really be comfortable though.
Would probably only do one clown in that sized thank. Two would probably max you out. Personally would rather have a bit more of a variety.