You just incriminated yourself by putting this up here!
Im sure we all have seen tangs in "tanks they're never supposed to be in" because they're too small, but they all seem to survive ok and seem normal. My buddy had a 2" blue tang in a 60 cube that just never grew, but seemed normal health wise.
Personally I've had tangs in small-ish tanks for years without issue, until I recently saw their need for swimming space and I felt bad. Upgraded to a longer tank, and now I want an even longer tank! I haven't seen monster yellow or purple tangs so I would imagine you'd be ok.