the tank is 20gallons and tall.
All the more reason not to put an angelfish in there. I understand they like tall tanks, but when a fish can have a body length of 4-6inches fully grown, a 20g is just too small. And I'm not even considering how tall these fish can get. Very impressive fish, but for a much larger tank, IMO. I would stick with tetras and maybe a small group of rams. A group of otos are also quite nice.
With the height in mind, you could probably get away with some of the smaller species of Echinodorus.
Echinodorus bolivianus- 4.7inches
Echinodorus horemanii- 10inches
Echinodorus parviflorus- 10inches
Echinodorus uruguayensis- 10-12 inches or more
Echinodorus "Red Flame" or "Ozelot green" or Echinodorus palaefolius var. latifolius - 16inches
There are more, but these should satisfy a search for now. I do not have much experience growing Amazon swords. I think I had 1-2 plants way back when I started this hobby 18 years ago. They didn't do well. I didn't know what I know now and I didn't have enough light or nutrients. Amazon swords appreciate a good substrate and the addition of root tabs. They also like a good source of iron. Under the right conditions, they can become very unruly. IMO, anything but the smallest specimens would be too large for a 20g aquarium. As for your other plants, Alternantera reineckii does pretty well with lower light levels and CO2 injection. I grow mine at 1.4 WPG. Bacopa also does well, but I would put the Bacopa in the background of your tank, along with the A. reineckii. Then, have a small specimen Echinodorus or two in the midground. Your foreground is limited. Dwarf hairgrass is tricky to grow. It is tricky because it is a relatively slow grower and prone to being covered with algae. Not a great plant for a beginner. I don't know what your plans are for this tank regarding lighting and CO2, so I can only make an educated guess.
An easy plant to consider from the region is Hydrocotyle leucocephala (Brazilian pennywort). It is virtually indestructable and will do well with lower light levels and possibly no CO2 injection. It is an easy plant to base and entire scape upon and you would really need little else to create an "Amazonian" feel to the tank.
If you're considering lower light plants, moss is always an excellent choice for the foreground. Moss pretty much grows everywhere in the world, so even if your specific species of moss wasn't from S. American, you would still be ok. Really, if you're into low-maintenance, a tank with just Pennywort, moss, some great pieces of wood, and a school of tetras would really be all you need.
Sorry for the length, hope this helps a little.
llj