as long as you buy them small, i dont see a problem with having a larger kind of tang but when it grows you will eventually have to get rid of it. maybe you can go to your Local fish store (LFS) and trade for smaller ones. just dont expect to get full price for what you think its worth. but i trade fish with my LFS every now and then so it should be ok to do what you want to do with your tank.
Do not listen to ths advice, that's a terrible idea. First off, there are more rwasons than just the fact that the fish gets too big, some fish, especially tangs, need the room to swim.
Also, habe you even tried to capture a tang/angel in a tank full of rock? There s a reason LFS keep those fish in bare tanks
Also, this is cruel to an animal. If you wanted a Mastiff and lived in a studio apt, you should just get a puppy, then drop it at a shelter? Didn't think so
Stock the tank you have correct the first time, save the trouble you are bound to have later.
Also, check out Brissletooth tangs, pretty sure they are a small tank species.