Minis aren't my thing but I do like appaloosas, I used to ride an appy mare about 13' when I was a kid, bolted first time I sat on it and the owner's kids didn't like it so it was the one I got to go on when I was allowed to share, and I came to love her and appreciate the depth of an apparently stroppy alpha mare when you bond with one

Last thing I did age 12 when we left my childhood home after dad died was go hug that mare goodbye and I never saw her again..
Anyway, to your posts. First British bred isn't the same as native breed, and minis that fine are quite high maintenance compared to a native, may well require a bit of mix or sugar beet and chaff at least til he gets a bit of weight on. But if he's been pushed off the hay supply at home then I would let him have decent hay to himself and start with what he has already been getting in the way of mix as moving stress, new home worming and new company will be more than enough for him to deal with and you need him up to weight/fitness for castration before you put him anywhere near the friend's mare as he is already fertile enough to serve her, as I'm sure you know.
Best of luck, I've been there with wee Welsh A colts off the hill I felt sorry for and took on in the past, all sold on a few years later to great ridden homes once they'd been rehab handled and backed etc but just a caution as he's entire, check there are 2 descended testicles before you buy as I have had 2 of a bunch of 3 with only one descended at 18 months and it's £1000 or more to extract the second one surgically and a rig isn't fun for anyone to live with.. I learned that lesson the hard way, particularly with wee ponies always check first!