If the rocks are large enough--gives them weight, and remember in water they are not as heavy as in air so they will more easily topple under water than in air--and rough and flat-like rather than smooth or rounded, they can fit together fairly securely. I used limestone rocks like this years ago for rift lake cichlids. Lowest rocks on the tank bottom and build up, then add the gravel.
Silicone will hold them together, and is certainly recomended for smaller, smoother rocks that would easily slip. Glue them in place and leave for 24 hours before adding water.