A mustard tang is Acanthurus Guttatus. I've never had one as they aren't my cup of tea, so I can't comment on their aggression level. But I can say you'll run into problems with those fish in a 200 gallon tank. The purple is aggressive in it's own right and the sohal will be just bad news all the way around.
From the web: "But in a small tank owing to their aggressive nature are violent towards not only with other species but also fish of same family and sex. For this reason they are also known as "Fish with a flick-knife". Since the major reason of dispute is territory space it is always advisable to provide a large aquarium. The Mustard Guttatus Tang should always be kept in quarantine for at least a couple of weeks to protect them against any infection, when introduced in a tank When frightened, strained or attacked by predators these fish extend their sharp blade like spines on their tail outwards. So be careful while handling this fish."
