Female bettas can be kept together although it is not advised for beginners or anyone not good with recognizing betta behavior. If you do keep females together in a sorority setting like Giz is doing here, lots and lots of plants and other things to block line of sight is mandatory. As well as lots of hiding places like caves. A 10 gallon tank is bare minimum size with no less than 5 females together to minimize aggression. It is also easier to use sisters from a breeder since most are kept in a sorority setting growing out which helps in some instances with aggression.
I get most of my bettas from other breeders. Not sure if the plant question was directed at me or Giz but I tend to use a lot of different plants that do not require high lighting or CO2. Several hygrophilia species, swords, moss, floaters, vallisineria, crypts, ludwigia, and apontagens are some of the different plants that I use. I sometimes pick up plants from my LFS but most times I order plants online.