We recently setup a 20g planted tank for the kids that's been cycled and holds:
1-black molly(pregnant)
1-white balloon molly
2-black skirt tetras
3-neon tetras
2-guppies
2-albino cories
I told my wife that I wanted to put a male betta in there and she flipped her lid about how it would eat the other fish and I'd end up with three crying toddlers.
So after reading about the sororities; my Petco just restocked and I found 2 ct girls and 2 vt's. The plan was to throw them in there and find an excuse to get another tank.
After the first hour my largest vt and smallest ct were constantly doing the tango and nipping- it ended with the vt looking like at double tail, but I think they've decided who is alpha(vt). The other two ladies will spread their fins but just swim away when followed.
I know it seems overcrowded, but after watching everything for several days everything seems to work. Everyone plays well together. The sorority actual sleep together in the ludwigia cuba on the right. Each will pick a spot next to each other for the night. The cuba also creates a nice calm area where I can feed them as well. The girls also seem to like the flow from the filter and will ride the stream.
...apologies for the ugly blue paper background...trying to remedy that- any suggestions?

cheers