Is it a long or tall 20 gal? The long has a bigger footprint and that makes it a better choice for the fish you're keeping. But regardless, I think you're right that you'd be overstocked if you added to your schools of fish.

You didn't mention what size or type of filtration you use on your 20 gal. Tank or how much water you believe is displaced by gravel and decor. It is a good idea to buy a filter that is rated 2x your estimated water volume. As long as your tank has cycled; you clean your tank once a week changing 25% of the water, your current stocking level is fine and can be increased. Your listed live bearers are hardy and can tolerate less than perfect water. The black skirt tetras and cory cats are also in that category. Neons are fragile and are very sensitive to water quality.
If your Pleco is of the common variety, it grows very large very fast. It also generates a lot of waste, putting a huge burden on your filtration system. For little tanks small Pleco varieties such as the "bristlenose" work best. As for your schooling fish you could double their numbers and still be fine as long as proper filtration and tank matenence is performed. Having 6 neons is not exactly like raising 6 oscars.