I've got naso, blue, yellow, and Kole tangs in my 180 (6x2x2) tank. They're all medium-sized without issues so far. They're all different colors/shapes/species to minimize fighting, with plenty of live rock for hiding spaces, and adequate skimming/filtration. I can imagine fitting another small or medium-sized tang in my tank if I really wanted to push it, but 4 is pretty much maxed out in terms of space IMO (even though the Kole tang won't get too large).