Actually Mandarin dragonets are more ich resistant then most fish. I wouldn't go as far to say that they cannot get ich, though their highly toxic, thick slime coat prevents parasite intrusion from nearly all external parasites. Again they are not parasite-proof but just more resistant then most fish.
As for qt'ing Mandarinfish I would highly recommend against it. Mandarins do not take prepared foods and get their meals almost solely off of live rock. Copepods, Amphipods, and other natural crustaceans that live on live rock are there main food sources and without a well established tank, lots of live rock, and a good refugium a mandarin will likely slowly starve in captivity.
I would get the fish home from the LFS
and into a well established reef tank as quickly as possible if you hope to keep it fed and healthy.