Betta must live alone.
They are hard wired genetically to fight....they do so in the wild, they do so as sport in several Asian countries. Their other name is Siamese Fighting Fish...kinda gives their nature away really.
That cute little Betta...male or female, makes no odds...can be the most docile creature for weeks or months but once it fixes on a target tankmate, that tankmate will die....might not be tonight ot this week...but it will die.
Betta have a very specific set of body language signals...
Curving tail & fins around floating food to stop other fish from eating, this moves onto doing the same with sinking food
Staying stock still and allowing other fish to swim past, then without warning snapping around and grabbing their fins or tails
Feeding time, all other tankmates stay near or at the bottom, tails to the Betta, stock still....a sure sign of the Betta bullying and starving them, also a general drop in tankmate activity and unwillingness to schoal anywhere near the Betta
Mysterious injuries on tankmates...tails, fins and eye area....thats your last warning before that fish will be dead very soon, usually overnight with the target fish is dozing
If you MUST put a Betta with tankmates....always have a spare cycled aquarium on standby (not for the victim(s) but for the Betta). Never place tankmates with long flowing tails or fins, a Betta (male or female) cannot differentiate tween species and they will spot the flowing parts and assume intruder on their territory and they will attack and kill it.
Never use any medication with the name "FIX" on the label as that will severely damage the labrynth organ on a Betta (same for all surface breathers) and can be fatal. Melafix etc contains tee tree oil which destroys that organ thus suffocation will happen.
Basically only very experienced fishkeepers should consider a female sorority since despite what the person in the petshop tells you, a female can be just as Jeckyll & Hyde as the male.
They are not beginner fish imho. They are the potential Pitbull of the aquarium and once a Betta has attacked and killed once, he or she will never tolerate tankmates ever again and should always be kept alone.