If your going to add a refugium, make sure you add something large enough to really do some good. Most of these small, 5 gal or less, hang on the tank, refugiums fall more into the category of toys, and don't really do much in the way of pod production, or nitrate removal by growing macro algae.
Consider something about 1/4 to 1/3 the size of the main display tank for ideal results, although something a bit smaller would work.
That being said, the tank size your planning, even without a refugium, should be large enough to support a mandarin, provided you don't have any other heavy pod feeders such as wrasses.