Careful with guppies in your tank; they are known to be able to cross-breed with mollies. The resulting fry can be very weak, have birth defects, etc.
Platies may be a good idea, but platies cross breed with swordtails. It's also worth noting that in my personal experience, swordtails just don't do well in 20 gallon tanks. My swordtail, as soon as I took him out of a quarantine tank, started growing and growing. He got pretty big and became quite the bully. He started shredding up guppy fins during feeding, and started chasing my rummy noses, ripping their poor fins as well. I had to rehome him, since he got very competitive for food and just became really angsty. He was constantly pacing around, clearly showing signs of being cramped in too small a tank.
My favorite tetras are rummy nose tetras. You can keep a small shoal in there if you'd like. Neons are nice, but they are prone to diseases. You could get cardinal tetras too, but they are wild caught and take longer to break into a tank. After they get comfortable in a tank, they are much hardier than neons.
I'm suggesting small tetras simply because I like small tetras and I've never kept larger tetras. Bigger tetras, of course, are always an option, but I don't have experience with them. Plus, if you get a mini-shoal of small tetras, you might have space to get something else for your tank.
Keep us updated!