I had 2 females living together for almost a year without fights, and then suddenly they started to flare at each other and within a week one was dead, she was chasing the smaller one and didnt see the glass, so after that I added 3 ghost shrimp, they lived together with carlota (the "smaller" survivor) for about 5 months, until 2 of the shrimps started to hatch eggs and carlota would try to nipp the eggs, seems like she got a taste for shrimp because the next morning I only found 1 shell and the 3 shrimps were gone, now carlota lives in a 20 gl tank by herself, Iam going to relocate her to a 2gl tank and use the 20 gl for adf's and guess what? yes!! 5 or 6 female bettas, but carlota is not invited to the sorority cause i dont want to be a suplier of victims for her killing instincts.
my sorority started with 6 but old age got the best of all but 2. Janice and Carmella were equally dominant over the other 4 but now they live peacefully together with Captain the plecko in 10 gal tank. They are of perfect health, but I certainly don't suggest started with only 2. 4 or 5 seem best in 10 gallons
I personally love having my plecko. Alot of people tried to discourage me about getting him. Honestly my girls and him got along perfectly and with regular filter changes his bioload is really manageable. They do get rather large, mine will be moved to 20 gal soon. I think you would be fine. Just make sure to get one slightly bigger than your girls because they MAY nip at him to begin with.