Honestly a 10 gal isn't that big. All of my 10 gal tanks are just betta tanks. I'm planning to set one up as a breeding colony of red cherry shrimp, tho. You could do a few guppies in there, but most fish that can live comfortable in a 10 gal require more upkeep than your average fish.
The 20 gal is another story. Those extra few inches of room really make a difference. Personally I'd do something like a shoal (12-15 fish) of a smaller tetra like neons or glowlights and then something cute and active on a sand bottom like dwarf loaches or panda cories. A large piece of driftwood makes a nice centerpiece and with a single 6500k daylight bulb plant it with some stem plants, dwarf swords, and some dwarf sag. Maybe an anubias or java fern attached to the driftwood.