As long as your new tangs are bigger than your established tangs, they would do fine. Don't buy any smaller as they will definitely get beat up. Buy and enjoy any of your favorite tang fish for now and don't worry about them outgrowing your tank. You never know if you would still be in the hobby a year from now or your fish are still alive after a few months or even weeks. Many people shut their tanks down in a few months, some, a couple of years ... etc... due to unexpected life circumstances. If you happen to still have your tank and fish in a couple of years, upgrade it to a larger tank or sell off your big fish, or trade them in or even bring them back to your local fish stores. That's what I've done for many years in both fresh and saltwater.