I love sororities- just keep in mind that there is always a risk, and even if everything is going good, down the road- months or even years- it can blow up and you may end up having to separate most, or all of them. Females can be just as aggressive as males and then some.
15 gallons is a great size for them- firstly set it up, and have it cycled (here is an easy guide to cycling)
. You can use live or fake plants- your choice. What you are looking for when decorating is making sure there are plenty of hiding places, and lines of sight being disrupted.
A good number of tall plants, ones that go to the top of the water if you can. Place them back to front of the tank in random places.. so a few in the back, a couple in the middle, etc.
You will also want some medium sized plants, also spread back to front to fill in what the taller plants aren't covering.
What a lot of people go by is if you look at your tank from the side, you won't be able to see the other side very easily.. and that you will have to look some to find all your girls.
A few caves would be good as well, as some may prefer making their territory in a broken terracotta pot rather then in the plants. Or you can use half buried coffee mugs (I find mine don't care for large caves made from coffee mugs unless they are at least half buried), or even glass/ceramic candle holders.
Once you have a good amount of cover and your tank is all set up you can get your girls.
Some say you can introduce 3 at a time, with a short time between adding in more, but to me it's safer to add them in all together.
The tail type doesn't matter- so you can use VTs, RTs, CTs, HMs, etc. If you are going to a breeder or buying some (I can give you the name of a breeder who is real good, wonderful fish and a very good price) online then I would get sisters- if not then it's not going to make that much of a difference.. as even some don't like their siblings.
If you buy them individually then buy them all at once- and introduce them all at once. Float them at the same time in the tank and acclimate them all, and then release them all together. At first they won't know what to do- but keep an eye in case one or two get very aggressive. You can take out the aggressive one, cup her and float her again for a few more minutes until the other ones have settled and try again.
You will have to keep a close eye on them for the first week at least.
You will see some chasing and fin nipping- let them do that as they will establish hierarchy among themselves. Just don't allow one to become excessive to the point where physical injury happens (other then fin biting). If one is over aggressive days later still, then you can remove her, rearrange the tank, reintroduce her and see if that helps. If not, then you may have to give her her own tank, or return her for another girl.
There will be a dominant female for the tank, so don't mistake her showing her stature rather then her being overly aggressive.
In a 15 gallon tank with enough hiding places I would say you can safely have 4-12 girls- you can also have just 4 girls and a few other friendly fish in there if you don't want just bettas.
You just want to make sure that if something happens, you can home some separately for good (if it's too late to trade in for another girl to try).
Here is a vid of a good sorority set up. Video
Good luck with yours!