I have ten or eleven females in a 10 gallon sorority. Aside from one female who is moving out because of issues with aggression (she continually harasses two of the other females) everyone gets on fairly peacefully.
I have never had issues with introducing females later down the track. In fact I added two the other day and they already know their place in the pecking order. I just feed all the original females in one spot and release the new ones out at the other end. Of course they do show up with the odd torn fin, but none of my females have ever actively pursued and attacked newcomers.
To feed my females I get them to all come over to me, and then I dump a line of pellets in front of them. This way the pellets aren't focused in one spot and everyone gets a chance to get fed. I've found the more well-fed my females are, the less fighting that goes on. I feed frozen foods with tweezers, but do pretty much the same sort of thing.
I never quarantine plants and I have never had any issues because of it. I would recommend quarantining new females however, and giving them a basic treatment for internal parasites as I have yet to get a betta from my LFS that didn't have worms.
The most important thing is providing appropriate cover for your females. My tank is still filling in at the bottom, but at the top it is extremely thick. I use riccia, water sprite, milfoil and wisteria, all of which require only decent lighting and the occasional dose of fertiliser.
This is a picture of my tub sorority I had running for a couple of months until I gave it away to someone on another forum. This still wasn't as much cover as I would have liked, so I used pieces of PVC pipe as hides for the bottom levels of the tank.
I never cycled my original sorority, but instead did daily 25% water changes for the first couple of weeks until my plants had established themselves properly. If you are not confident with live plants, it would probably be better to fishlessly cycle your tank before adding females. Living in a sorority can be stressful enough, and you don't want to compound that with poor water conditions.
My sorority has been running for 7-8 months now and I have never had any issues with it. Because I have a penchant for collecting females, it's handy that I don't need to have individual tanks for all of them. I have a 100 gallon tank sitting on my bedroom floor that is begging to be turned into a sorority.