If you are going for a mini reef, then you will need more rock for any corals that you decide to add to the tank. The rock also helps as a natural biological filter and it helps to cycle the tank quicker. If you are going the fish only route, then you don't need any live rock in there at all. Is that the plain nano or the DX model? If it is the plain model, then you might not have enough light in there for corals. If it is the DX model, then you might have just enough light for medium to low light corals. I have seen some guys that have retro fitted the hoods with around 128W of light for their mini reefs.