Sororities are a female only thing and for a 10 gallon you would have 4-5 females together and that's it in the tank.
The easy but not always accurate way to tell the gender of your betta is the fins, Long are male and short are female. This gets complicated though when you consider the existence of things like plakats, young and not fully grown fins, damaged fins. Pictures would help.
If you go with tetras I would say 4, maybe 5 but that's pushing it. A good site once you get the hang of it is http://www.aqadvisor.com/
Since all species have their own requirements the number you can add will vary from species to species. Most small ones like tetras and platies you'll be looking at 3-5 max fish.
Another option if you like the idea of a second betta would be to divide your 10 gallons. Most pet stores sell tank dividers. A 10 gallon can be divided into a max of 4 sections basically making 4 2.5 gallon tanks in one. I personally wouldn't recommend more than 3 sections and suggest only sticking to 2 sections if you do go divided. However it would let you have several male Bettas in the same tank without them killing each other.