hmmmm, tang, are you looking for the algae aspect, or the color and size
if it is the algae, i would look at lawnmower, sailfin or algae blennies(same species), and can be added early, they are hearty fish
if not, you may wish to consider a dwarf angel(later on, after the tank has matured several months), but in either case, i too would consider not adding a tang
chromis are great little fish and i have never noticed any aggression from mine, and they werre the first we purchased for our 40 gallon, nd some of the first moved to the 75 after we got that running
or like mr salty said a clown is another good choice, but i would go with a percula or false percula, since they are the least aggressive, and in tank your size you would probably want more smaller and less aggressive fish
HTH