If you do want to start a sorority, make sure you do plenty of research first. :) Make it heavily planted, then put some more plants in. And them some more. Heavy planting is the key to success. Have spare tanks standing by in case it goes wrong. Cup all your girls, including the one you currently have, and re-arrange the tank before adding them.
If you'd prefer to do just a normal community tank, you have quite a few options in a 20 gallon, especially with a female. Here are some ideas:
- tetras: cardinal, neon, glowlight, green or gold neon, ember, rummynose, pristella, lemon, penguin.
- guppies (a bit hit and miss)
- Endlers livebearers (the males are very peaceful)
- loaches: kuhli or pakistani/botia/chain/reticulated (all the same name for one loach)
- bristlenose pleco
- otos (beware, they are quite sensitive, so be prepared to lose at least half of all the ones you buy)
These are just some of the more common options. Things to stay away from are goldfish, cichlids (this includes angelfish), gouramis and nippy fish like serpae tetras and tiger barbs. :)
In my sorority tank I have cories, ember tetras and Endlers Livebearers and everyone gets along brilliantly, so I can recommend those breeds from experience. :)