Hi! Your boys are lucky to have found such a caring owner!
Guinea pigs are a lot like humans when it comes to pairing up - the chemistry between them needs to be right; so yes, acceptance can fail and there can be tussles or even bloody fights. However, acceptance in young sub-adult piggies is generally very high as they have their instincts and their body chemistry urging them to pair up. It is getting more difficult the older sows past their ideal child bearing age are (sadly they never have a menopause) while boars tend to mellow with age.
Since it is the sows that accept the boars, I would recommend to start with the most dominant sow; then bond your most dominant boy with one of the more submissive sows, so you get the two trickiest bonds settled first and have the option to make another attempt with a different partner. Take the time to only conduct one bonding at any time and always provide new and clean fleece or towels for each new bonding combination. It pays to not rush it and to get every pair right, starting with the two most difficult piggies to pair up, so you have got the most options for them.
Here is our very detailed bonding guide:
Bonding: Illustrated Dominance Behaviours And Dynamics
Allow the boy and sow time together in a divided bonding pen to get to know each other, ideally overnight. The more you can take the extraneous stress factors out, the better. Make sure that there is something to eat in the middle (hay or grass) and that you can refill, as well as acouple of water bottles. No hideys!
Brace yourself that a highly sexed hormonal boy can completely go overboard and lose the on-switch for his brain for a day or two and just mount a sow whichever way. The bonding can still come off as acceptance has happened, but you may need to give the piggies a breather in between. Unlike boar bondings, you can put cross gender bondings on hold once acceptance and the worst of the hierarchy sort-out have happened and a bond has been established.
Here is a classic example of a failed bonding where a rampant boar is overlooking all warning signs from more dominant sows. I knew that neither party is aggressive, so I have allowed the bonding to progress as far as tussle with no injuries for the sake of making an instructive video:
(PS: Barri now lives with 3 other sows and is taking life a lot easier these days while Hyfryd has still not accepted even a submissive boar after her bad past, but the sows live next to a pair of bonded neutered boars. )
Neutering is not going to change much in the way a boy is behaving; they are still going through the teenage months and react just like full boars. A lot of testosterone is produced in the urine, so conduct any bondings in a room you can open a window or outdoors in the cooler hours of the day. It can be a stinky pee fest from both sides - the boars making the girls as 'theirs' and the sows target peeing to keep the boys away from their relevant parts if the are not in season and if they have not submitted to a boar's mounting right at the beginning (they only will if they are also submitting leadership to him); a dominant sow will keep a submissive boy at bay. Things can get rather lively and vocal, and you have to sit through that, even minor tussles when a sow tries to protect her bum from a rampant boy. If he doesn't accept when the limit is reached, that is when a bonding can fail.
This guide here can give you a bit of an idea of just how dramatic it can get, especially when a sow comes into season spontaneously and during the first few seasons in a new bond - the drama actually serves to cement the bond.
It is important that you have no hideys at all until things have calmed down. Just peg a sheet or blanket over the bonding pen or cage and at the worst put a temporary divider back if that becomes necessary.
When Sows Experience A Strong Season (videos)
Sow dominance behaviour also applies to mixed gender groups:
Dominance Behaviours In Guinea Pigs
I hope that that helps you?
