Hi and welcome!
Fights between sows are very rare; unlike boars even if they have fall-outs, they manifests as mouthfuls of hair rather than full-on bites or as rather sneaky, but systematic bullying. If your girls are getting on well after all that time, you are however VERY unlikely to end up with a fall-out or an outsider problem.
If there are problems, the introduction of a neutered boar will not mend them. A boar is part of the overall group and has got his place in it, but he is not part of the (separate) sow hierarchy; a boar is in many ways a natural outsider. If for whatever reason a sow decides that another one is no longer part of "US" (which, as I have said is RARE), then it is much wiser to split the group and find the single piggy a new companion. Once piggies have made up their mind as to who is "US" and who is "NOT US", they don't change their mind.
Neutered boars only lose the ability to make babies, but they do not lose their natural behaviour. You still see plenty of rumblestrutting and mounting when sows are in season (but it can also be the sows mounting the boars). Boars can still generate an amazing amount of boar stink with their testosterone laden urine, with which they generally spray sows during an introduction. However, what neutered boars (we call them "husboars") bring to a group, is that they can act as a kind of glue. In order to mate with a sow in season, he has to get on with them all. The hanky panky during a season is acting as a kind of emotional purge for the group as a whole; it keeps the bitching between the sows down more. Fall-outs between bonded husboars and sows are extremely rare.
However, if you want to add a husboar to a group of sows, the big hurdle is the initial acceptance, mostly by the top sow. For that, I very strongly recommend dating your girls at one of our recommended rescues that has got neutered boars, so the piggies can decide whether they gel or not, and you do not run the risk of ending up with a boy that doesn't get on.
I have and have had a number of mixed gender groups with a husboar each; both dominant leaders of a group and submissive lads, depending on how dominant the top sow is. The main thing is that they work out, and that is something you have to try, you can never predict it (and I say that after about a half century for successful and failed cross gender bondings). I currently have got four husboars living with 1-5 sows each, but at one point, I had up to 13 sows living with a husboar. And yes, I have taken my sows to a rescue for husboar dating on several occasions to make sure that they were working out.
Age is not an issue compared to mutual acceptance. Husboars and sows of all ages can live happily together. As a point in case, my 3 year old Carwyn was first bonded with a frail 7 year old lady who he was very kind with, but after her demise, he was then introduced to a 5 week old baby girl and her 6 months old companion; it was instant love on first sight between Heini and Carwyn, and that has not changed 6 months later; they still have regular snogging and popcorning sessions together!
Carwyn with old lady Ffraid
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Carwyn with his new "wifelet" Heini
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You may find these links here helpful:
Introducing And Re-introducing Guinea Pigs
Illustrated Bonding Behaviours And Dynamics
Sow Behaviour