Just in case you decide that you're all for a sorority, and are willing to lose the other fish, I'd like to be of some help
I had my sorority of twelve in a 30g tank, but disease set in even after a very long quarantine period and I ended up with six, just buying the seventh. They live in a 10g tank successfully and somewhat peacefully. I'll give it to you straight- a sorority is very stressful on both you and the fish. You never know how the girls will react, and you never know what's going to happen in the end, who's going to end up hurt or dead. The first few weeks will be absolute heck to watch as they set up a pecking order (from Alpha female and downwards) and you'll see a lot of gorgeous tails get shredded.
One of the most important things about a sorority: PLANTS, PLANTS, PLANTS! Live, fake, a mix- it doesn't matter. All that matters is TONS of plants and TONS of hiding places. I have more than 30 plants in my 10g, and too many hiding places to count. With my girls, the sorority is still strong, but illness is still a possibility, and I dedicate a lot of time in my day to watching them and making sure everyone is healthy.
With your fish, if they all come from the same small locally owned business, I would only quarantine the girl already in the tank and then after two weeks add her in. If they come from big chain stores (PetSmart, PetCo, etc) I would quarantine them all for two weeks. You'll need back up space for all of the fish, in case someone doesn't get along. Keeping them in separate tanks, giving them to friends. You may also want to ask if you can extend the warranty on the fish, just in case after the two week period it doesn't work out.
All in all, sororities are very expensive and hard and it can take months of planning to get it just right. But. BUT. After all that, it can be such a rewarding experience and can brighten your day right up when you come home and all of the girls are swarming the front of the tank, flopping over each other just to see you and beg and plead with their little eyes for some extra pellets.
Oh, and with your 9g tank, I would make sure that if it's cycled and you do proper water changes and make sure you have a decent filter, you can have up to nine girls. I wouldn't put nine girls in there, but a maximum of six, as the more girls the less fighting, since the aggression is spread out.
Oohitsae, if you find anything I've said wrong, please let me know, as even after nearly eight months it's still a learning experience!