Kat, can i ask why so little substrate? Also if going heavily planted you would be limited to floating/plants on wood/rocks etc with no substrate?!
Because if for some reason the sand suddenly needs to be removed, it is easier to do so when there is only a little bit of it as it can by syphoned out during a normal water change. The only reason for wanting to do that which comes to mind at right this moment is some sort of algae problem.
No, I do not mean restricted to floaters

I pot the plants I keep in breeding tanks.. by pot, I mean that I sometimes use ceramic pots and sometimes I use the mesh pots (which plants sometimes come in). I add the plants to them, then tie to the side with some thread and weigh down with a stone.. complicated, but I find that this is easier to maintain than a fully planted breeding tank. Also, my breeding tanks are forever going up and down, depending on what I'm doing at the time, so keeping a permanently planted set-up is not an option. The plants come from my main display (I usually use Crypts, but then I've only ever had them melt once when they were being moved) and I just sell them off when I'm done with the tank.
I am not saying that the only way to do it, but I have found that dedicated breeding tanks work best if they are set up like that, with the plants filling 1/2 - 2/3 of the area (from one end of the tank) and the other end is clear with a few suitable spawning sites in the plants and out of the plants. This sort of set up is not much good for observing the fish, but it seems to make them feel most at home.