I have a Kole tang in a 90 gallon and he does fine. He is grazing on algea all the time and is healthy. I see no problems with the smaller tangs in a 72 or larger tank even when they are full grown. In fact many on this board gave me that advice when I was stocking the tank.
I have a hawaiian trigger, Kole tang, coral beauty, maroon clown, and diamond goby in a 90 and they are not over crowded.