I was planning on having my rock over a 2-3" layer of sand. This would give the advantages of sand but not have the sand compressed. I really want to have the added biome of sand.

You can have some sand in the front but by using the liverock...sand is un necessary you can use *some* but 2-3 in....?? I wouldn't bother. Unless as in the book you *primarily* use sand to make up the Lbs per gal and less rock Ok but not both Just my opinion Look at my survey, see the substrate *most* people use and FWIW only a few there are readers of mine

I have a 200g built in to a wall that is visible from both sides. How wide front to back? Left to right?

I intend on piling live rock down the middle to make a mound and cave structure that is visible from both sides. I feel like I can only put about 100lbs of rock in here before I lose the view through effect. The question I have is should I fluff the system out with plastic media since I can't put enough rock in? or should I still lose my plastic and just use the sand and rock as bio filters?

There are a few approaches

1. use very large pcs for the structure Dinner plate or larger, a good supplier will be able to get large sizes for you connect the live rock with plastic dowels (inexpensive clear) these can be found at a plastic manufacturer Yellowpages plastic/sheets/rods Use a drill and carbide bit drill and connect. This method is very labor intensive but will produce a nice effect. Also for the long run allow for maintenance ie:takedown if needed
2. use a couple cubes of eggcrate and stack the rock on that. Here I am of the opinion that you don’t want or need "baserock" that will just give you pockets for detritus to accumulate

here are some sources

If you call tell them I sent you, especially Jack he will accommodate youModern Plastics 800-243-9696 ask for Raul Flores

That it will do. Also I describe a "unique method of aqua scaping" which uses eggcrate and a spraybar This is a *glorified* Berlin method and my favorite

I have about 15-20 lbs. of sand left over. I would like to place it in my sump. It would be like a denitrater/refugia. What do you think?

If you have enough live rock you shouldn't need a de nitrator. As far as refugia yea...but only if you have enough room not in there with other pumps and in bags etc. A *sperate* sump for actual refugia w/o being cramped some minimal lighting where stuff can be left undisturbed to actually grow ok Personally I don't like stuff in sump for reasons as ... being unnecessary...taking away water volume from the sump, building up unseen detritus and who knows what. Unless you have a very large sump area...I wouldn't do it. You just don't need it.