You can keep female bettas together as long as it is under the following conditions:
1) there must be at least 3 to even out the aggression
2) they must be well fed--a hungry betta is a nippier betta
3) it must be at least 10 gallons of tank, but a planted 20L is best IMO so there's room to swim away and hide.
As far as housing bettas in general...
They need a minimum of 1 gallon of tank. But as with most fish, the bigger the better. Sure, they can survive in a gallon, but they won't be happy. Check out the difference between a betta in a 1 gallon jar and one in a 10 gallon tank--the one in the 10 gallon is 99% more likely to be more active because they have more room to move.
My perferred method for keeping my bettas is in a divided 10 gallon tank. Put some plants along the divider so that they can escape the vision of the other fish, but leave an opening so that your bettas can flare at each other when they want. To be healthy, they need the exercise of flaring so it does do some good. If you have a betta that is a scaredy cat, then completely block the divider because it will just stress itself out.
I hope that this helps.
EDIT: It also helps to keep in mind that each fish--whether it be male or female--has its own individual personality. So treat them accordingly. For instance, some bettas do well in community tanks, others don't. You won't know for sure until you get your fish as to what it will like. But what I have recommended has worked for my bettas with ranging temperaments.
On the heater note: If your tank is 2-5 gallons and you are using a heater, make sure you check the temp regularly as in such a small tank the temp can spike up very rapidly. It must be an adjustable heater and usually you don't have to set it to the temp you want--usually set it a little bit lower. I recommend testing the heater's reliability for a couple days before adding the fish to make sure the temp is stable.
, also u'll beable to enjoy watching him swim about enjoyin the room.
