How old is your tank? If you don't have a HUGE supply of copopods, I wouldn't recommend getting a mandarin. They will not take prepared foods while juveniles, and most end up starving to death. Supplementing them from a refugium or with live brine can help, but it's still risky. Make sure the specimen doesn't have a sunken tummy--should be fat and sassy, constantly picking at rock for hidden copos.
That said, appropriate tankmates are those that won't compete with the mandarin for copos--mostly, larger, carnivorous fish, that also won't go after the mandarin. Not much choice--I know. We have one that' just recently started taking prepared foods after about 18 months. He was in the invert tank (only fish in there) and then in the FO with a small lion, a trigger, and a snow flake eel. Shrimps are okay, as are most crabs, if you opt not to have any other fish.