Parasites can infect the gills of a mandarin just as they would any other fish and and most definitely can bring parasites in with them to a display tank. I know this for a fact as I have inadvertently introduce parasite to a tank with a mandarin addition.
Ich will not usually be visible on the the skin of a mandarin due to its slime coating but this dos not mean the fish is not carrying the parasite in its gills where the parasite kills the fish by suffocation.
IMO mandarins will not live in the average home aquarium unless food trained. Your small qt is the perfect place to do this training. Mix frozen mysis with live brine. If the mandarin wont even take live brine you may want to return him.
Check melvs reef mandarin diner for an some excellent info on mandarin diet concerns when even in large community displays. IMO the fish is not particularly difficult to maintain in any tank as long as the proper efforts are made to verify its dietary needs. As you will see if you visit the mentioned site they can even be trained to take dry food offerings.