[glampka]

You want 1-2#/gal of LR depending on its' density. As far as stacking it goes - put it in the tank 1st so it is on a secure base. It's protection against burrowing fish. I don't like putting the rock against the back glass for a number of reasons - 1. usually ends up with dead zones for circulation. Detritus will build up there. 2 makes it difficult for cleaning. 3. fish will tend to hide back there. 4. If a big fish dies back there you have to tear 1/2 the tank apart to get it out. Be sure the rock is stable - you can epoxy pieces together if you need to. You want to have caves, tunnels & the like for the fish to swim through as well as to hide if threatened.
As stated above, forget the tang & mandarin. Clams & anemones require special lighting so be forewarned.