I wouldnt use silicone for the live rock, i would use it for the dry base rock, and stuck together when the rock is dry. Reason is silicone takes a day or two to cure, and if cured underwater, it releases some chemicals that arent good for fish tanks.
You can pick up like 8-10 dollar sticks of aquarium safe epoxy at your lfs, these would be a better choice since they are aquarium made and will cure safely underwater.
You can pick up like 8-10 dollar sticks of aquarium safe epoxy at your lfs, these would be a better choice since they are aquarium made and will cure safely underwater.