IMO you want at least 5 girls. It has been my experience that 4 girls tend to have more aggression issues. Personally I think 7 is a good number in a 10 gallon, however that is over stocking. With a sorority you definitely what to have the tank cycled before adding the fish and you should also do two weekly water changes IMO. I used to do one 50% and one 10-25% each week.
LOTS and LOTS of plants. I can't stress this enough. You shouldn't be able to see the back wall of the tank it has so many plants. Caves aren't nearly as important as plants. You want them tall enough to break up the line of sight at the top of the tank since that is where bettas spend a great deal of their time.
Here's a picture of one of my old 10 gallon sororities. To be perfectly honest, after having sororities in a 10 then a 20 and then a 29 I think 20 gallons (long) is a much better size for a small sorority than ten, but it is doable in a 10.
Also remember... there is a good likelihood that it may not work out with one or all of the fish. And even if they seem ok at first doesn't mean they won't turn on each other down the line. So always be sure that you have enough tanks, heaters, etc to house each fish separately in case they decide to murder each other (believe me, it happens >.<).
I'm not trying to dissuade you, I just want you to keep in mind that many sororities don't work out so you have to have a back up plan.