Common Minnow, Flathead Minnow, Bitterlings, Rainbow Dace, Weather Loaches, Garra pingi pingi are just a few examples, the only problem is finding them so its best to see whats available to you.
As you mentioned cories I thought I would mention tat I used to keep peppered cories in a coldwater tank. From information I founf on them, including Planet Catfish, they actually prefer cooler tempatures and tropical tempatures are actually at the top end of their tolerance.
they dont produce much bioload, and its is a 20 gallon, so i dont see why not. and they really do like to be kept in huge groups. Maybe you can get away with 6.
Singles are a big NO I would say three for now .... aenus corys are so hyper You may decide you want some other corys too go with them eventually ... They really are great I have 3 in my 10 gal.