It depends on the layer of gravel you want. About 20-30 should make a 1-1.5 inch layer of gravel. You can possibly keep a few cichlids, but not too many. Go with about 5 Mbuna.
I am making a planted discus tank with a sand substrate. It will have discus, tetras, corydora catfish, and then I want to get a stingray. Would this set up actually occur in an Amazon River basin?
Sand can also work. With my friend's cichlid tank, the parrotfish just eat a bit of substrate and burp it out again. Just find some aquarium sand. As long as it doesn't affect the water chemistry, it's good.