Personally ... I wouldn't stick a tang in a tank that small, unless you plan to upgrade in about a years time, or sell him off when he will inevitably outgrow that system ... from my experience ... they grow from the size of a half dollar to the size of a tennis ball in about a year. Just a thought ... I haven't experienced a friendly tang either ... they get VERY territorial.
I had a yellow tang <--- my sig. He did very well! And was pretty big... Here is a shot for scale. 54 Gallon Corner. Tang police are going to get me but he did fine with my Hippo Tang. He only got aggressive with my Jawfish... which really didn't make sense lol.
what is that i hear....oh no siren of the tang police....lol
many people will tell you story of how they keep tang in "smallish" tank. i too had kept a purple tang in a 45 gal. but over time, the more i learn about this hobby, i would not keep another in such a small tank. imo there's no magic number of what size tank can a certain species of tang can be kept in. the bigger the better. if one decides to keep it in a "smaller" tank, be prepare for some potential problems to arise. be it aggression, pacing, ich, wasting away, hole in the head, etc., if one is able to recognize and resolve the situation then more power to you. it's just easier on the tang, the tank mates, and your piece of mind if one would keep a smaller tang in a bigger tank.
listen to suggestions with an open mind but ultimately think of the fish well being. sure we all can live in a closet if we have to (like some manhattanites......lol) but we wouldn't be happy.
They are not going to reach max size in captivity and IME grow very slowly compared to other tangs I've had. They are readily available at small sizes too.
this story is an exception to the norm. from what i've heard, koles are not agressive. some are not so even with other tangs. but my friend and i each had a kole that were absolutely nasty. i witnessed his cut up a radiant wrasse pretty bad. and mine chased my blennies every chance it gets. we both returned them to lfs.