I like buenos aires tetras. They are cheap, relatively clean and get decent sized. Plus, they don't outgrow a 20 gallon tank. I have them in with various polys (the biggest is a 6" senegal), and none get eaten. I think you will have water quality problems with your current filters and regimen of water changes. Doing a 20% water change 2-3 times a week will keep water parameters more stable. You will DEFINATELY need a bigger tank soon.