I would recommend against either of the tangs. They just need too much swimming space that you don't have. Furthermore, the small hippo tangs have a horrible survival record, I never get one less than 3-4 inches long. Be patient on the mandarin. Yours is a new tank, and doesn't have enough Copepods in it yet. It takes about a year for a tank to begin copepod production. You could pour Copepods into the tank, but at $25.00 a dose (figure 2-3 doses per week), you will be better off spending your money elsewhere. Your plan to exchange the yellow tang with a lfs is a common mistake. Fish don't come when called. Frequently you will end up tearing down a tank to capture a fish, which is very traumatic to the fish being caught, but all of your other stock too. Add fish based on their adult size to avoid this problem. Sorry to be a "Debbie-downer", but in the past 40 years of fish keeping I have made all of the mistakes you are considering.