People get way too upset over this topic for some reason. Tangs are the best 100% reef safe algae cleaner. Please tell me how you tell a fish is "happy?" If the fish is healthy that's about your only indicator. Any comparison to their natural environment is just ridiculous. Sure I would love to live in a mansion, but I do just fine renting a room in a small house in the suburbs. Even that comparison is dumb. The size of the fish at purchase has a lot to do with the "appropriate" size. I've owned quite a few tangs, and the only one that's ever posted noticeable growth within 1 year was my Powder Blue Tang. My favorite comment of all (which I am surprised that I haven't seen on this thread) is... that tank is too small, get a rabbitfish. LOL
It's a judgment call by the owner. That's all. Decide on a tang you like, then look for people that own that fish to get opinions on the growth rate, and activity level. Now if you were trying to stick some type of Naso or Blue Hippo in that tank... that's a completely different story. Those guys are all known to grow extremely fast and get VERY large.
I've got a 2-1/2" Eibli Mimic Tang in my nano-ish SPS tank. He's fat, healthy, free of threats from predators and disease. If he does experience mutant growth (which I highly doubt)... then I've got plenty of friends who have offered to take him, and there's certainly nothing wrong with that (it's not a puppy... it's a fish). However, the most likely solution is that it will be with me for a number of years as I upgrade and move to larger systems, never outgrowing a single one.
Opinions are like... well you know... everyone's got one, and... ;)