If your tank already has a good amount of live rock in it then you dont need to buy anymore live rock at all. The tank should be mature though at atleast 5-6 months old. It would help if we knew what lighting you have right now on the tank. Make sure you have good water peramaters too. The usual ones that people start with are the mushrooms, they dont require as much light, and are relatively easy to maintain for the most part. For placement, it depends on the coral, some types like low placment high water movement, others high placement, and lower water movement. Find some that you like, and before you buy them ask about them on the board again. HTH
Sea Goblin
