I have read 2-3 Gallons per Betta is required, plus a ton of hiding spots, at least one for each fish. The hiding places should also be all over the tank, not just in a corner or along the back wall. Since this will make it harder for them to make a quick getaway.
Also, you should introduce all females to the tank at the same time, so no one fish has had a chance to claim anything. Fighting shouldn't be too rough. If there is a particular girl that is a bully, try separating her for a few days, then reintroduce. If this fails more than 2/3x, you should find her a new home.