You know Tangs are pretty active fish so I have no suggestion as to a type that is small enough for a tank that size. I wonder though because it seems like fish in smaller tanks seem to be more timid. I had a small tank when I first started and the fish were always hiding and were not as healthy. When I put my first set of fish in my 180 they were shy for a couple of days and then they hardly ever hid. They'll eat out of my hand and if I have my hand in the tank to get something or clean the glass they'll just follow it around the tank like they are curious or something. Even my Hippo tang is not shy at all. Sometimes the friendliness of the fish can even be annoying its like they are face invaders. When you put your face up to the tank to look at corals they get in front of you like they want attention or something.
Seems like gobys, clowns, damsels, dwarf lions and other smaller fish that don't need a lot of swimming area would be best for that size of tank. A hippo tang would certainly be miserable in that tank when It grew up mine is about 6" now it would die in a tank that size.
I have seen this shy fish small tank pattern even in freshwater fish, I wonder if fish are clousterphobic (sp error).