And if the tank isn't currently cycled yet, it shouldn't hurt anything. Your spikes will be higher because of die-off and the like that stilllearnin said, but in the end it'll probably speed up the cycle.
Look around online, there are a lot of great places to get live rock from. It puts that stuff you see in your lfs to shame.
http://www.tbsaltwater.com
Tampa Bay Saltwater, they have some awesome live rock and the prices are even reasonable. I think with shipping and everything it's something like $6 per pound, but I can't remember.