It is not recommended to have a sorority in a community tank, just as it is a horrible idea to keep a male and a female together. However, that tank you mentioned has a load of problems I've already mentioned in your other thread, so I won't get into that now.
I don't recommend baby bettas either as they can be difficult to sex when they are very young and you still don't know their personalities. It is best to go to a breeder. I can't remember the name of the breeder I used, but when I think of it, I'll let you know. I was very satisfied. There are several reasons to buy from a breeder instead of your LFS. With a breeder (well, a good one) you'll know the origins of the fish, so you know the lines, you know that the breeder knows the fish well and can give you a good estimate on temperament and general health. Breeders also often run sororities because it is easier to keep female juvies in a tank together than separating all males and females into individual containers. The breeder runs the sororities, isolate the ones who are too aggressive to be kept in a community setting, and then you can buy females which are already used to each other. WildBetta and RCA both can provide some good info on keeping the species in such a way (I don't remember if WildBetta has a sorority, but WB is well versed in the species, and RCA has a successful sorority).
However, if I were you, I'd work on that larger tank's stocking before stocking your 55. You may need it to hold some of the fish that really are not suitable for the stocking in that tank.