Somewhat depending upon the species, it can work. A harem of one male and 2-3 females. Or a pair. Again, depends upon species. However, most will tell you that Apistogramma are always better with other fish, dither fish; a 20g doesn't have much room. Many years ago I had a harem of five A. bitaeniata in a 15g (same substrate area, which is what matters, as the 20g high) and managed to raise fry.
Given the natural habitats of all Apistogramma (there may be an exception, unknown to me), I would forget the rock caves and have lots and lots of wood. Wood and leaf litter over sand is the apisto aquascape. And floating plants to darken the tank; Java Moss and Fern is fine as well.
Byron Hosking, BMus, MA
Vancouver, BC, Canada
Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]