Keep in mind while your aqua-scaping, that most fish (that I've seen) enjoy swimming into caves, and around structures. It gives them something to do all day to keep from getting bored. It's much better than having a plain tank.
Also, some fish like to move around the substrates like sand, and this will cause your rock to shift. So make sure the rock is secure and solid.
I will try to post a picture tomorrow to elaborate on the whole caves thing.