Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Sand will not stay on top of gravel. It will sift between.
I have grown a lot of plants in gravel, but never the little ground cover plants. Best growth was Vals and several species of Echinodorus (Swords). Any of the non-rooted plants are fine, they are grown on wood or rocks: Java Fern, Anubias, Bolbitis. And floating plants are fine, too: Frogbit, Floating Fern, Salvinia, Water Lettuce, Duckweed, Azolla, Water Hyacinth and more. (Some of these are high light or pond plants, not great in aquariums)
Put some root tabs in there and most plants will be fine. Use water column dosing for the plants that are not rooted in the substrate.