Be careful on the stability. Just wonderin' if they are stable right now. I put 2 hands in the water. One hand to give a moderate but good nudge to the rock and the other hand to catch it if it does fall. I get to feel real good when things don't move easily. May have to play a bit to get a good set-up. Also make sure they are firmly seated on the bottom for when you get that pistol shrimp and watchman goby (OK, if you get one). Sicklidkid suggests using the larger ones on the bottom. If you lay a couple of'm with the lond side down, that'd create nice little burrow paths under the sand for the shrimp too. Heck, even if no pistol shrimp, I too think you'll have better stability with the larger ones lower.
Also consider where the powerheads might be and think about dead spots if you can. Then again, you might not be able to see those until you start feeding. Bottom line, you'll probably move all the rocks 3 or 4 times anyway