If you put the rock on top of the sand you can have some stability problems (especially if you get any animals that burrow). Here is what I did, I bought dry base rock for the part that would be buried under the sand. Then I put in dry sand, you do not need to buy all live sand, just buy about 25# of LS to seed the rest of the sandbed, your sand bed should be around 4-5", Then you can put the live rock on top of your baserock structure and you won't have to worry about rocks toppling later on. You should put it all in at the same time and start your cycle, if you add them seperately you could proglong the cycle.