I would definitely ditch the damsels if I were you, they get very agressive later and will be harder to catch once your tank is filled with rock. Even if the lfs gives you live sand, mandarins eat a LOT of live food, and I am pretty sure you will not sustain the mandarinfish for more than 6months. As for the tang, he'll need a bigger tank in a few years, but it's not too big of a deal right now.
How many pounds of live rock do you have in your tank?