Hey Adam, welcome to RS (again...I caught ya on your other thread
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In my opinion, it is better to have as much live rock in your tank as you can afford. The live rock will serve as your bacterial filtration...ideally you would want 1-2 pounds per gallon (sorry about using american measures, let me know if you need me to convert to metric). If you are looking for ways to raise the live rock to appear as you have more, you can make racks out of PVC piping with holes drilled in them (in order to keep water from becoming stagnant). Also, I've read that some people use PVC as pylons - stuck vertically into their sand/substrate, while the rock rests on top...this will also elevate the rock, and will allow for better flow under as well.
I did have a question....when you use the term coral sand....are you referring to crushed coral or aragonite sand? Crushed coral tends to trap detritus if not maintained and will keep nitrate levels elevated.
As far as the type of silicone....I'm unsure....I'm not very handy - that's what I love about this board - lots of help. I'll search around and see what I can come up with.
Hope this helps!