You're going to be told two things on your post. 1.) Your tank it way to small for any kind of tang and 2.) If you have to have one you'll need to get ride of it once it starts growing into adult size. But, the Yellow Tang will be the smallest as an adult I think 8in. but the others that I like are the Blue grows to 12in. the Naso 18in. and the Powder Blue 9in. it's more about the room that they need to swim. Tangs love the open water where they have alot of room to swim...just my 2cents.
Jeff