Ideal Gravel Depth for Carpeting plants

[Blaha]

I have this tank idea. It is an inkling of an idea. I would share its progress when set up began, but it is in the planning stage. Probs a few months out at least because I still change stuff on my 29G and have no need to start another project yet.
I have a spare 14 gallon tank (I believe 20L x 10W x 18D) and a laser cutter. 
The idea is to cut shelves that would span from the wall to about 6 inches from either side of the tank, essentially splitting the tank into 3rds with the left and right having a higher tier and a cave-ish underneath them. They would be 4-5 inches tall, which would be enough space to fit a small sponge filter underneath them, and the air outlet would go through a hole in the shelf. It would be a fine grain black substrate, as in not normal pea gravel size.
I think it would be really cool to put said substrate on the shelves and plant some Micro Sword or Dwarf Hairgrass on the shelves, and in the little caves, now shaded immensely by the substrate above some small wood or rocks and Anubias Nana or Nana Petite, and some stem plant either floating or planted in the center, where there is no shelf.
It would then be a guppy community tank. Maybe some corys and a bristlenose or something else like that.
There would be a lot of work in the design of the shelves and the selection of the acrylic to cut and method of attaching the sheets of acrylic together to make something that is appealing to look at but also Strong enough to bear the weight and safe for the fish. Maintenance on the sponges would likely be difficult but probably worth it.
My question is how deep should the substrate on said shelves be at a minimum to support good health and growth of the Micro Sword or Hairgrass? I would have to make walls for the edges of the shelves to ensure the substrate doesn't just fall off and also make sure the shelves could take the weight but that part wouldn't be hard.

[gardenman]

You could probably get by with three-quarters of an inch to an inch of substrate. An issue you'll have with corys or a bristlenose is them moving the substrate out of the trays if you use a fine black substrate. Finer, sand-like substrates don't tend to stay where you put them if you've got corys or a bristlenose. Any acrylic should support the substrate in such a setup. Just the typical window replacement acrylic available at a hardware store should be adequate for the shelves. Just a note also, you can buy premade acrylic trays in a variety of sizes for less than it would cost you to buy the acrylic and solvent/adhesive. Just Google "acrylic trays" and you'll find a gazillion options in every size, shape, and configuration imaginable. Women often use them to organize jewelry and makeup supplies.

[Blaha]

I had not thought about the corys and bristlenose moving the substrate. That is a good point. I love corys which is why I'd want them, and while I've had plecos in the past I've never had a bristlenose. My corys in my 29G do sometimes do a little digging.