There's a simple way to prevent siphon back on your return pump. Drill a hole in the nozzle below the water level. This way, when/if it siphons back, once the hole becomes exposed to the air, it will break the siphon. Still a good idea to keep it very near the water surface, however.
weight does not include the substrate weight, theyre 2 seperate items.
If you do want to add live rock over time, the only trick is making sure it's cured. If it isn't cured, the dieoff from shpment or whatever can cause ammonia spikes. Allowing it to cure before you put it in will ensure this isn't an issue.