With a very few exceptions, what most people mean by "centerpiece fish" does not work in a tank less than 3 feet/90 cm in length.
I also agree with others above who suggest leaving well enough alone...though I would say that there certainly is room for another fish species provided it is the right match. Neons remain in the lower half. Otos you don't see if there are lots of plants, but they are all over the tank and don't really count when considering "space" in the tank. I would look for a small "nano" type fish that will most probably be a shoaling/schooling species, which means a group of 10-12. There are many possible, and these do tend to remain in the upper half. Ember Tetras, or one of the dwarf rasboras in Boraras. A group of Nannostomus eques, a pencilfish that is not bright in colour but its oblique swimming at the surface and extreme peacefulness make it a winner in any tank. Considering the bottom, a group of pygmy coriesis another possible, if you have sand.
This is only dealing with numbers and species. No mention has been made of the water parameters which might affect these species. And 20g is it a standard (24 inches/60 cm length) or a long (30 inches, 75 cm length)?