Play sand is great!! Probably better than gravel. As ADFs go to the surface for air and then high-tail it to the bottom (this is an instinct that protects them from predators in their natural habitat. They are much safer near the bottom/in brush/caves/hiding areas). The sand is far better for them when they swim full speed into it. Remember, most fish will more likely than not have a sand or mud "floor" than gravel. Any rounded, non-sharp gravel will be fine though. That cave size sounds okay, they don't need anything large. I find I have a group that hides in the very small place between one of my broad leaf plants and the sand. This brings up another point, be careful with rocks/decorations that may be too heavy. ADFs tend to dig or try to dig under things to hide. Any decor that is too heavy can potentially crush them or trap them under water, causing them to suffocate. Just make sure any large things are burried well or can't easily be dug under. I have some river rocks in my ADF aquarium, and have never had any trouble.
\Dan
