Fine black gravel will work, if you don't mind the expense (this stuff is quite expensive, though a 20g will perhaps not break the piggy-bank like a 4 or 5-foot tank would
); a grain size of 1-2 mm. Coarse sand is fine, I now have it in 5 of my 7 tanks, and I use Quikrete play sand from Home Depot; a bag at $6 will give you plenty. If you can get black pool filter sand, fine; but not the normal white which is way too bright for the fish. Quikrete Play Sand is basically identical in appearance to the natural Amazonian stream sands.
I agree with losing the indicated fish; the Bolivian Ram may also be problematic in so small a space. Mine is in the 5-foot 115g tank and he definitely rules that space.
Plants. With a single T8 "daylight" tube with around 6500K, you will be OK with pygmy chain sword, any of the small crypt species (several are in the profiles), Anubias, Java Fern, Java Moss. Brazilian Pennywort is a distinctive stem plant that will manage with low/moderate light. Water Sprite floating is ideal.