The 20g high space is limiting, but with your water parameters (well suited to characins) I would look into hatchetfish and pencilfish. These all remain smallish, are quiet (not active swimmers), so they suit small spaces better.
Among the hatchetfish, stay with the species in Carnegiella, there are 4 and all are in our profiles. I have these in all my Amazonian tanks.
Among pencilfish, any species except Nannostomus beckfordi will work. N. beckfordi are rather feisty and can get nippy esp in small spaces. But all the others are peaceful and colourful. N. eques swims at an angle, so that is something different, and a group of 7-8 would be fine. N. marginatus, the dwarf, is quite pretty and maxes out at about an inch. These two are more often seen than the other species.
The Ember Tetra is small, and a group of 8-9 would be fine with the Lemons.
A Twig Catfish and a Whiptail Catfish would both work (individually or together), and a group of 5 corys, or 6 with 3 of two species.