We have had our small standard, Chico, since he was a weanling. He’s 13 now.
I would not be comfortable with a mini donkey out with the horses- they are teensy. Our small standard can - and does- hold his own with my boisterous gelding. I’m 5’5", he comes to about my waist, however many hands that is LOL…he is not intimidated and in fact, when our newest gelding was settling in, he foolishly tried to push Chico around rudely. Chico, reared up, grabbed Scout by the crest and bore down hard. Scout, all 15.2 hands of grown, 9 YO horse, squealed like a terrified girl and his knees buckled- Chico let go when Scout was about to hit the ground. Yeah, no, the horses will not get away with beating up on a small standard. Those critters are tough.
The jacks should be gelded- they can and will get quite nasty tempered as they age.
Ours foundered lightly a few years back- like ponies, they are such easy keepers it can be hard to keep founder at bay. Chico wears a grazing muzzle as needed and just keeps a wee bit of oats or some such to keep him busy while the big boys eat their feed. I have to watch him, he can and does share their meals- they just let him eat with him. Toppy won’t but just about every other horse that’s been through here, he’s worked his donkey magic on them and he gets to eat their food.
He gets the same vaccs and deworming protocols as the horses, his feet get done every other time.