Hi! Could you please clarify - Do have already got a boar? Is he neutered or not? Or are you just planning on on introducing a neutered boar to your herd?
In the latter case, have you been looking at already neutered boars in rescues that allow you to date him at the rescue for acceptance, so that you can nicely side-step the tricky issue of operation and post-op wait and will be able to introduce straight away? If you can travel a bit further to a rescue that offers intros at the rescue under expert supervision, you can make sure that you come home only with a boar that is going to fit with your girls and that you have the rescue to fall back on if things don't go quite to plan. With cross gender bondings, age is secondary to mutual liking/initial acceptance by the sows.
Guinea Pig Rescue Centre Locator
If you are considering getting a full boar for neutering by you, please be aware that you need to find either a general vet with plenty of experience in guinea pig neutering or an exotics vet with practice in small furries operations in order to minimise the otherwise still high risk of especially post-op neutering.
You have also to take into consideration that you need to conduct a full 6 weeks post-op wait until a boar is 100% to go with sows, and that acceptance is not necessarily guaranteed. I have the surprise daughter of a supposedly safe over 5 weeks post-op boar (not one of mine) living with me, just to prove the last point - it can really happen!
Guinea Pig Vet Locator
Please be aware that while a neutered boar can act as glue to a sow herd in soaking up grumblings when they are in season to a certain degree, he won't be able to make sows that don't get on do so. That is asking too much, as he is not part of the sow hierarchy itself - and he will stay scrupulously out of any disputes that involve the sow hierarchy.