regarding starboard under sand - I like putting something down there to spread out the contact points of the rock and to provide some protection against a dropped rock. I am confident this is not necessary, but I do it anyway. I used to use plexiglass, but stuff could work under it over time. on my current tank, I used white floor tile - it was cheap, easy to cut, and I siliconed it down. I kept the edges away from the tank seams.
But this is purely a preference or opinion that gives me some extra confidence, lots of people don't take this step with no apparent risk - and as I allude, you always have the chance of adding risk if you do something unusual - could the extra layer expand or move against a silicone seam, could it release a toxin, could it trap detritus underneath, etc.