My hippo was only maybe two inches when I got him and now is about 6 inches at least and is really fat. He has been the same size pretty much for the last 2-3 years out of almost four years that I;ve had him. He grew pretty fast by my standards. It was like one day he was tiny and the next he was big lol.
I personally wouldnt put tangs in that size tank, but again thats just me. We cant force you not to add one but can only give you advice.
If you like the yellow of the yellow tang how about an angel? The angel I had was brighter yellow than my tang was ! lol