Live rock has a porous surface and lots of surface area for nitrifying bacteria to anchor to. It helps in the cycling process by breaking down ammonia into nitrite and nitrite into nitrate. With enough live rock and a deep, poorly oxygenated sand bed, you can develop denitrifying bacteria that will help break your nitrates down.
If you've added live rock, I wouldn't add any fish until your ammonia and nitrite levels are at 0. It doesn't just waste money, it's unethical in the fact that it needlessly kills fish. Besides, most damsels get aggressive towards your other fish anyway. With a good amount of live rock and some patience the tank will cycle in no time.