If the local SPCAs cannot help then I'm not sure what to suggest but others may have some knowledge there.
As for them not getting on, well sometimes they decide straight away, and you'll know it with their teeth-chattering and getting worse until they fight. Fights can happen suddenly, or can build up. It's best to separate before that point but sometimes it all happens so quickly! I'd take some thick gloves along in case this happens. They can bite you by mistake so gardening gloves or similar are a must.
Sometimes they keep testing each other, in fact often they do, and this can either calm down or escalate. It can be a slow build-up to acceptance or rejection. Sometimes there can be a bit of teeth chattering and you think it will fail, and then it calms down. I found in my last one, one of the sows began teeth chattering for a minute or two, however my boar didn't chatter back and things settled down into quite a smooth introduction. At that point though I was thinking it would end up in failure. It was the fact that my boar backed away from the situation (which he has never done before) that saved the day and they are now very harmonious. Some coriander distraction may have helped too. It had helped that the sow in question had behaved submissively at the start of the introduction, and I think Freddie the boar had already got into his head that he was boss at that point so perhaps didn't view the sow's chatter as a challenge. I don't know.
I don't always find it easy to tell from straight away if it will go well or not. The ones I've done involving teens and adults have always been tense to begin with. I've left mine for several hours together before feeling confident enough to place them into the cage together (with one exception - bonding my newest sows to my recently bereaved sow went smoothly pretty much from the start).
It's good to bring some food along and their favourite treat to distract them if things start getting a bit worrying, but they do also need time to sort themselves out without interference, if that makes sense? I diffused the sow-to-boar chatter with some coriander, then let them get on with it and all was calmer. All in all I consider that a good introduction! Perhaps others have had much better ones
I have had sows offer nothing but reassurance to babies being introduced into the herd. Other sows have chased and 'bullied' the new baby for a day or two.
Babies are often accepted but not always, and sometimes when baby hits the hormone phase two pigs can fall out.
Sometimes if you find a submissive adult, it can be the way to go as by then the pig is likely to stay submissive. A good introduction can be where one humps the other, and the other one doesn't try to hump back. It's likely to work out. When they both hump/challenge each other things can take a while to settle or escalate into failure. I guess you will know a good one when you see it, a bad one too, but the ones that can go either way are hard to judge for a while.
xx
