From reading up on the fish on WWM, it is on the hardier side of butterflies, but that still doesn't mean that they're bullet-proof. They're also supposed to be on the more reef-safe end of butterflies, but it sounds like anemones are their favorite, followed by SPS and then LPS. I wouldn't know how it would be adding them to a tank with established tangs, but it's supposed to do well if added before. Also, they do better in pairs or groups than solo.