No mangroves! Those are trees. I wouldn't want to have that deep of a sand bed either. I would go 1-2 inches deep. If you plan to use a DSB in the sump, you should put the cheatomorpha(or other macroalgae) in another compartment. I don't have first hand experience, but I've heard that when you cover a DSB with algae, you build up tons of detritus. You should put some kind of small pump in the DSB chamber to create a circle motion going above the SB, hit the glass and go into the SB, go through the SB, then come out on the side of the pump; repeat. I hear this keeps circulation through the SB and because there is nothing on top of the SB, you end up with no detritus build-up.

As for stocking, hold off on fish and most coral until Thanksgiving passes. I would say add a few hardy red mushroom corals and see if they are growing, thriving, and reproducing. I'm sure there are things for a CUC to eat. I would add a small CUC like a few hermit crabs, a few cerith snails, a few nassarius snails, and a few nerite snails.
Just remember, no SPS, amemones, or giant clams until your tank is about half a year old.

Good luck and happy reefing!