I really like my platies. They are active and don't just eat the food at the top but will graze the bottom for food also. I already had some of the orange one with the black tail that were given to me. Then I found some mickey mouse platies that are a mix of yellow orange red and black. I also have 3 peper cory's and some shrimp in my 20h.
I also agree, not enough swimming room. Personally I would want a 3' minimum length for them. I suggest white clouds for your tank. They are very under rated IMO. You could easily have 15 in there.
Update: Just wanted to thank everyone for their help. I went down to the LFS just to check everything out and see what caught my eye, and wound up falling in love with the Black Phantom Tetras. I got 3 of them to start, and so far they are doing very well. I plan on building up a school of 6 black phantoms and 6 serpae tetras. I think these fish are a much better choice for a smaller tank like my 20G.
I just can't decide if I should get 1 more school of some kind of tetra, or a couple bottom feeders in there to round out the tank....I guess I'll have some time to think about it though.