Not sure about the butterfly, but the sohal is an amazing fish. They're relatively hardy, but they will get large and will become the most dominant fish in the tank. It'd probably do well in a 210, but take care when selecting other fish.
Thanks everyone for all your input. I already planned on the Sohal being the last fish in the tank because of thier aggression. Personally I would rather have and Achilles Tang but my husband got is heart set on a Sohal. I'm still trying to find out more about the Golden Butterflys. Its going to be a fish only tank so I'm not concerned about them being reef safe, I'm just kind of wondering how difficult they are.