Damsels are extremely territorial by nature. If this is the typical advice given out by your LFS I'd not ask for any more advice from them at all.... the quick 'sale' and out the door obviously bears more importance to them.

There are far more humane methods of starting the nitrogen cycle than subjecting fish to the toxic environment of a cycle. If you take it to mind to return the fish (maybe for store credit?) you can use a raw peeled cocktail shrimp instead, giving you some time to research fish temperment.