At my work, we have a 535 tank with a Clown (5"), Powder Blue (4"), White-Tailed (thompsoni, 3.5"), and Dussumieri (8"). In my two years of employment there has only ever been one conflict that left any damage, on the Dussumieri and Powder Blue. They are in there with many larger tangs and other fish, so their aggression is heavily diffused by other fish in their way.
These fish have decided that they will get along, thankfully. I'm sure if a tang were to be added, it would be murdered. I think the mixing of several tangs is highly possible, if you don't pick the ones with similar colors, identical (perfectly) shapes.
You could do an Achilles, but not with a Powder Brown or Powder Blue. I don't think that would work at all. A Sohal might work, if it doesn't decide to kill the Achilles.