I personally love Purple tangs, and have seen them relatively small so you would probably be able to find one!
And just to back you up, I can't stand when people are rude about someones tank set up. I am one to have made a mistake but when I am yelled at on a forum about THINKING about purchasing a specific fish, that is just absurd. I completely agree that it is only responsible to help another hobbyist when they may be doing something that is a detriment to a fish or their tank... but choose your words people!
Wish you luck in your future fish. Things in those smaller tanks can be difficult but you seem to know what is going on.
You should put up some pics!