Only small experience, but we have four girls and a (obviously neutered) boy, and I think our boy stabilizes the group. He is the most mild mannered, he doesn't try to mount the girls, he just is a lovely, friendly placid pig who spends all day doing the Wiggly Bum dance, as he's so happy to see his girls!
I don't know if all Boy Piggles are the same, but I think he's our friendliest pig. And our combo works very well. I wish you could keep loads of boys together, as I'd have thousands!
So from my perspective I'd recommend a boy with girls, when we introduced him it was amazingly easy, like he'd always been there, no territorial behaviour AT ALL! Flob and Daisy were just like, oh, Hello! When we introduced the two girl babies (they must have been about 4 months old, the others were about a year at that point) it was much more stressful. They are all happy together now, but I think boy and girls works really well.
I've found that carefully introduced sows are ok in pairs, trios, sometimes more but for larger groups or groups that change regularly, a neutered boar is worth his weight in gold. It makes the group much more stable, stops the girls from strolling at each other... I've had a group of 12 or more living very peacefully with our neutered boar Paddy
Hmmmm - I've kept several herds of piggies over the years. My groups have always been very stable with or without a neutered male and I've been able to remove piggies and add new ones with no problems at all. If you have very dominant females within the group then a neutered male is a godsend and will help to keep the peace but he does need to be a strong willed boy - not a wuss ! I've had groups of just a few - say 4 or 5 and sometimes anything up to 20.
I have a few friends who have also tried this and have had nothing but trouble - fighting sows, smaller pigs being badly bullied and other rushing around and squealing terrified.