^ The above mentioned thread is a great one!
I just set up my sorority on Monday. I'll tell you that I decided NOT to go with petstore females, for several reasons, but mostly health reasons. If you have never ordered fish online from a breeder, you might be surprised to see that when you get your fish in a box in the mail, they actually look healthier than the sad little bettas on the shelf.
The biggest things to a sorority are quarantining - make sure none of them are sick. THIS IS SO IMPORTANT. One sick girl can wipe out your whole tank. Try to find a QT set up where the containers are clear and all of the girls can get used to the presence of the others.
Decor - plenty, as mentioned, to break up the line of sight and give them hiding spots. When one of my girls is being chased, she darts into the heavy plants I have in there and the aggressor stops chasing her once she is out of site.
Water parameters - if you don't have a ton of live plants, you need to fishless cycle the tank first. While a single betta can do okay during a fish-in cycle, the stress of the cycle combined with the stress of living with other girls can easily cause sickness and devastation. I have both live plants and a filter, and I'm testing the water EVERY day since I set up the tank until I am 100% sure that the water parameters are safe. Adding 5-6 girls at a time can cause a mini-spike even if the tank is already cycled.
Another thing that some have mentioned on that other thread is that if the filter current is stronger, it mitigates aggression by making it harder for the girls to stay in place when they are attempting to flare/attack.
But there's also, adding them all at the same time or very close together so that one can't establish territory before the others get the chance.
I will say that, when I first put my girls in, I saw almost immediate aggression. Do not freak out. You will likely see flaring, nipping, chasing, and maybe some tail slapping, but (this is also mentioned in that thread) the girls will be pretty good at sorting it out on their own. It is time to remove a girl or give a girl a time out if two attack each other and neither back down. If there is every any aggression in my tank, one of the girls ALWAYS darts away. There will be some ripped/torn fins and you may see some scale damage - another reason why the water needs to be perfect. I add vitachem to my tank to boost the girls immune systems and help with their torn fins.
However, they are very entertaining to watch, fun to feed, fun to get to know. I can sit here and watch my sorority tank for hours. The smallest girl in my tank is the most aggressive one, and she chases EVERYONE except for Mercy, my biggest girl. I think she may be the alpha, but Mercy is only aggressive towards those who are aggressive towards her first. She also "breaks up" some of the fights and keeps my small girl away from the others. It really is like a sorority, a little fishy soap opera. Sometimes I imagine them with voices and make up a little story as I watch them... But maybe that's just me, LOL.
I know that was a lot of rambling, but I hope it helps :) Good luck! Sororities are fun!