Brian--use agricultural pumice, it's very rough and irregular looking, not round or smooth. Look for it at larger well stocked nurseries. Make sure you get pumice and not perlite. Is perlite what you're thinking about? It is genuine crappola in my opinion and has no place in a bonsai pot
With respect to your root over rock situation, you have bought the tree already part of the way there. The information you are getting is for just starting one from scratch. Leave it be and pot the tree where the soil line is currently. Each year you re-pot this tree, you can expose a little more of the rock by potting the tree a little higher in the pot. Take it easy on yourself just now; I think you are getting ahead of yourself and being confused by what you're reading. The important thing for you to do just now is get this thing crankin' and fatten that trunk up.
Take red pine's advice about the watering cans. They are the best way to go if you can't use a hose. I just assumed you were going to grow this thing outdoors. You will have to put it outside if you want to do what you have indicated and that is to beef it up. That process is severely curtailed by keeping the tree indoors.