I'm using ADA Amazonia as a substrate. Is plant growth affected if I layer gravel or sand over it? (About an inch or so) Is it better to leave the substrate exposed? I'm also planning on mixing it with part Eco-Complete for better root hold with carpeting plants and thicker substrate layer.

ADA Amazona may be the best substrate available for growing plants. Trying to make it better by adding another substrate material is counter productive. If I wanted to get a thicker substrate, without the added cost of more Aquasoil, I would use something like Black Diamond blasting grit, but under the Aquasoil, not on top. Another option is mineralized topsoil under the Aquasoil.