In a 20g, you are limited to one dwarf cichlid species. With our water (Pacific NW, and I am up in Vancouver) being soft, it is perfect for the rarer species. And you will need some dither fish, or the Apistos may be too insecure and less healthy as a result.
We only (so far) have one Apisto in the profiles [you know about our fish and plant profiles, second tab from the left in the blue bar across the top of the page?], namely Apistogramma cacatuoides [click on shaded names for that profile], because this is probably the most common and most "adaptable" to harder water. Most of the data there applies to any apisto species, with a few exceptions. Some are best in a pair, some in a harem--if you can buy them with male/female obvious. Years ago I got hold of a group of five of what was then called Apistogramma kleii, and let them pair off. It was very interesting; one fish was a dominant male, the other 4 appeared to be females; the male spawned with one of them a few times. When he died, one of the other fish suddenly developed into a dominant male and then spawned with a female. Quite a surprise for me back in those days (early 1980's).
There are some other impressive dwarf cichlids too. The checkerboard species in Dicrossus, Dicrossus filamentosus and Dicrossus maculatus being the two more usually seen, are some of the most beautiful. The Wet Spot gets many of these in when they are "in season" from Peru, Colombia, etc.
For most of these you will want the water more acidic, but that is easy. With the very soft water the pH will naturally fall as the aquarium matures. My tap water is pH 7.0 to 7.2 with near-zero hardness, and if I leave things alone the pH in the tanks runs around 5. This is ideal for these fish, and their tankmates.
Pencilfish are superb, and/or the hatchetfish in Carnegiella. Cardinal tetra. Substrate fish are possible, but if you want a successful spawn (rear the fry) then I would avoid them, as they are the prime predators of eggs or fry at night when the cichlid parents are not "awake."
For info on the other two apistos you mention, try these: Umbrella Apisto (Apistogramma borellii) - Seriously Fish Apistogramma panduro - Seriously Fish
The A. panduro is strikingly similar to a species I had in the 1990's, Apistogramma nijsseni.