The things to watch for when getting live rock are:
Die Off (in an established tank with fish this could be a problem, best to cure it to be on the safe side)
Insane prices of LFS
(best to order live rock from www.liverocks.com
, you get better color at better prices with cool life)
Undesireables (before you put your rock into your tank shake it out in some salt water to shake off any red crabs or mantis shrimp
1-2# per gallon provides for excellent natural filtration. The more the merrier.
You could order 60% base rock 20% gulf rock and 20% keys rock from LR
.com and get great rock at great prices with great life on it.