This is a 10g tank which is really not large enough for most of these fish. A single male betta, fine; with shrimp, fine. But no fish.
Without the betta, nano fish will work here. Ember tetras, dwarf rasboras (like the "chili"), pygmy cories. But any of these in with a Betta is not advisable. I know some claim it can work, but we have had threads from people doing this who suddenly report that overnight the betta killed "x." It happens. And in reverse, a sedate long-fin betta is an easy target for small fish that can decide it is fun to fin nip. Putting fish in such situations just is not humane.
If you decide on nano fish, Quikrete Play Sand is the cheapest good sand for fish. Or one of the aquarium river sands, but not white or black. And it must be inert, so it will not increase GH/KH/pH.
Cories are shoaling, they need larger groups. If you go with the pygmy in the 10g, a group of 12-15. This fish does not do well in small groups. With this, you could have a group of 12 dwarf rasboras, or 11-12 Ember Tetras.