What blue damsel are you referring to? Chrysiptera cyanea? If so, I wouldn't attempt it. They are not a schooling species (or even shoaling, for that matter). The store may have had a lot in one compartment, but that is only a "bandage" for a long term problem. There will be eventual problems if it is attempted in the home aquarium. If the fish remained that way at the store (discounting the fact that they are constantly being removed, moved, as well as decorations being moved), a pecking order would develop and fish would begin to suffer. If you do want to keep a few,however, that is fine. They naturally consist of a loose group of one male and several (2-4) females. Just make sure the tank is large enough.