Reef,
Let me see, three tangs in a 210. That should work. You do know that the Chevron Tang does not stay the pretty orange color its whole life? It turns into a dull brownish color. I like the powder brown, but in the Acanthurus family, I am a fan of the Sohal - which will keep up with the aggressiveness of the Zebramosa family. Most of the Acanthurus family are quite fragile, specifically: Powder Blue / Brown, Achilles and Whitecheek. I have had poor luck with the purple tang, but other people will not indicate that they are super aggressive. Mine killed three of my prized fish about 8 months ago.
Let me know what you decide. BTW the Naso will get over a foot long. You did not mention the Regal Tang - they are in a family all their own and get along with other tangs just fine. Good luck.
-Dr Marco :sfish: