I generally agree with what has been posted about a mandarin. Yes, like other fish they can carry disease, so it's your choice. Quarantine it to make sure it doesn't have anything, or put it in the display tank right away so it can hunt pods.
You typically need about a 50 gal tank to produce enough pods for a mandarin. By the way, you must not have other fish that like to dine on pods such as wrasses in the tank. Yes, some of them will eat other foods. I've had them eat frozen bloodworms and live blackworms. In my opinion, any LFS that tells you they train all of them to eat anything is an [many expletives deleted] liar. They might get a few fish to eat many foods, but there are quite a few that will only eat pods. It's the luck of the draw.
One additional point. All those fantastic colors on a mandarin are there to camouflage the fish. Against the light background of a tank in a typical LFS, the fish will stand out. In a well developed reef system, with lots of coaraline algae and other things on the rocks, it will blend right in. You may have a tough time seeing it, unless you look for it. Sometimes your better off with a fish that will stay out in the open more.