I would be inclined to pick either rams or apistos, but not both in a 20L. They are different species, but close enough in size/shape, that they tend to not tolerate the other in constant close quarters. Being a planted tank, going with more than 1 female would be alright, but IME the male will tend to pair up with one and the other will remain a loner.
If you're dying to have both in the same setup, I was able to run a pair of each in a both a 6' long and a 4' long (both 18" wide) planted tank setup.