First thing, welcome to Tropical Fish Keeping forum.
That is a good combination of fish for a 20g, and lots of plants will absolutely help both fish species. I would add a couple rasbora, say 7, they do better with more. I assume you are thinking of the common Harlequin Rasbora. If you can find them, the other two species in the same genus are actually a bit better, the Lambchop Rasbora or my favourite the Hengels Rasbora. All three are in our profiles--you will notice the fish names shaded, meaning that species is in the profile so you can click on the name to see the profile.
On the rams, you will have to get a bonded pair. The males in both species of ram, Mikrogeophagus ramirezi (and all its colour varieties) and M. altispinosus, must select their own mate, and they bond for life. The best way to deal with this is to observe the fish in the store tank for several minutes. You will observe the males pushing and shoving each other, and if there is a female close to the male that is not pushed or shoved, the male is accepting her. Buy those two fish.
The other way to do this is to buy a group and let them pair off the same way, but that takes a larger tank and then you have the other fish to get rid of.
As for plants, I go simple. All my tanks are planted [photos are under "Aquariums" below my name on the left] and use the natural method. For what you are setting up, I would go with a coarse sand or fine gravel, dark, as the substrate. I use playsand, Quikrete Play Sand from Home Depot or Lowe's. Inexpensive and authentic. A 1.5 inch layer is sufficient.
Light is important, do you have a fluorescent (tube) fixture, or incandescent (screw-in bulb)? I can suggest good tubes/CFL bulbs for whichever.
Plants. We have several in the profiles. For this setup, the pygmy chain sword would be ideal. Another is the Dwarf Sword. Floating plants are essential, Water Sprite is the best, but Brazilian Pennywort would work here too, it is a stem plant so you could plant it in the substrate and/or have it floating. At the head of the Aquarium Plants section there is a 4-part series "A Basic Approach to the Natural Planted Aquarium" that may give you some general background.