I agree about white clouds - they were my first fish to spawn, and will always hold a spot in my heart and in my community tanks. I kept 3 in a 5gal once with a betta and they seemed to do fine, and actually that was the tank they spawned in. I still have several generations of them.
I am very interested in the badis as well, especially since you have posted about them, Menagerie, and though my favorite LFS
always has a label on a tank listing them, I can never seem to be there when they have any left!
You could probably keep Endler's livebearers in a 5gal hex, but maybe just 4-5 males or a trio of one male and two females (and babies