I think dwarf sag looks really good in smaller tanks, and it doesn't need a ridiculous amount of light or CO2. It's kind of tall and grassy. As far as mossy things, doesn't java moss and those weird little moss balls do fine without CO2?
With a school of tetras, you'll probably want some plants that are reasonably tall. Most small schooling fish really like some tall plants to play in. Of course, a truly tall plant like anacharis would be too tall for a 20 gallon unless you like to trim constantly or like dangly things everywhere.
If you want a 2nd school ... Some rasboras would probably school a little higher in the tank, but I don't know if anything that's truly a schooling fish would fit well in a 20 gal with a reasonably large school of tetras. You'll probably need to keep the schools pretty small if you do find something that works. If you like neon tetras, you'd probably be better off just getting a big school of them, then getting some bottom fish and maybe a few larger non-schooling fish.
The weird thing I've found about schooling fish is that eventually they will get comfortable and stop schooling if you don't have any semi-aggressive fish in there. So, you don't want anything that's going to eat them, but you might want a couple fish that'll chase them around now and then.
So I guess I'm mostly agreeing with hoppershaun here. If you got a dwarf gourami, it'd probably keep your tetras in school. I don't see where there'd be a problem having a pretty big school of tetras and 5 cories, since they're not really competing for space. Ottos can also fit in just about any tank, and they're really cute for sucker fish. You have to wait until you're sure your tank is stable for them, though, because they're pretty touchy, and you should wait for a little algae to develop before you get ottos. Cories will just root around the plants and gravel, and ottos will suck on the walls.
Of course, there are other centerpiece fish besides gouramis. I never liked gouramis until I saw a gold one with almost cheetah-like spots that I really liked. Since you aren't talking about any long-finned fish, you could get a betta, or a few swords/moons/platies, or who knows what else instead of the gourami.
If you want two schools, one thing that might work well is to have a small school of a bigger kind of tetra (like Lemons or Phantoms) then a larger school of neons. But say you have 5 Phantoms and 8 Neons ... on paper that's not overstocked at all, but you'd probably have to have a really good balance of plants and everything. I'd say just add the neons first and a few bottom fish and see how it's going after a few weeks.