Well there's a very good chance your tang is never going to get up to the advertised "max size", most people don't keep tanks that long. But it is something to think about, if a tang can get up to 12" long, do you really want your tank to be 6 body lengths? Tangs are swimming fish and grazing fish, so you'll need to have space for both, the problem with too many tangs (I have 4 in a 270) is they can very easily completely wipe out most of the algae in the tank, sure you'll see them picking here or there, but they're pretty good at what they do.
If it were me, I would go with a Ctenochaetus type (bristletooth, they tend to be more on the smaller side, they scrape rather than pick at algae), and maybe a zebrasoma (yellow, purple, black, etc, would not go with a sailfin though). Make sure they're well fed and aggression might be minimized, but if there is a lack of food watch out.
If you were to go with Acanthurus (powder brown) I would stick with only that as the king of the tank and leave it at that.
