Both the powder brown and the clown tang are "Acanthurus" and most tangs don't get along with their own genus, right? Additionally Marine Depot says :
"The Clown Tang is a medium/high maintenance fish and may act agressively toward other fish. It is not recommended to keep with other tangs. Oriental Surgeon. Though this is a commonly used species when small, this fish can become a very intolerant bully towards its tankmates, getting progressively worse with growth."
A small tank is only going to accentuate that and encourage the behavior sooner. They also like a lot of rockwork and hiding places. It doesn't sound like a good fit. If you're interested in keeping another type of tang in your tank you should look to a different genus.