Field of Dreams, this is exactly my point...Closer in size doesn't make a difference in some cases.Big on Big or Mini on Mini can even hurt each other. Even exact same sized horses can hurt each other.
My old QH mare was kicked in the jaw by her half sister (very closely matched and bred similarly on the non-related half) when they were youngsters, the other horse cornered her and slit an artery and cracked the jaw bone. That mare almost died in my arms in the pouring rain, but since I was standing in the barn and heard it happened we were able to get the vet out quickly enough that she lived 20+ more years. She lost so much weight you'd have thought she had been starved. She looked awful.
Her first colt died at about 9 months old from sticking his head in a fence, panicking then breaking his jaw in 3 places & puncturing his lung with a broken rib. No other horses were involved, he was pastured across the fence from others with no common fence lines even. The next foal born (different mare, few years later) ended up with a broken pelvis from her dam pushing her into a fence post (my sister was standing there and heard the crack when it happened) in a pen that I had completely redone before the mare foaled, baby proofed completely from terror of the last colt and his freak accident.
Yet the next one dropped her foal early and by surprise before we had brought her into the 'foaling pasture' and we found the filly in the middle of farm equipment from going under a non baby proofed gate That foal is still here and though I want to wring her neck most of the time and I'm knocking on wood saying this, oddly enough she's the kind that doesn't get into trouble. Her half brother, grandson to the mare that almost died in my arms, I love to pieces and he regularly tries to blind himself, in fact just this week he had another scratch near his eye.
Horses are an accident waiting to happen no matter how we bubble wrap them. The ones you like the most will be the ones that try and kill themselves daily. The ones you don't care for end up living forever without a scratch.
I've had more trouble with the wrong personalities than I have with size differences. That same mare I first mentioned went on to be 'auntie' to nearly every horse we ever weaned until she died, including the gelding I'm keeping with my B mini now. This was also a mare who would lift her feet to avoid kittens playing with her tail. She'd cock her head and check before she'd put her foot back down. As a kid I'd fall asleep in her stall at horse shows and they'd find me laying down with her. A total sweet heart of a mare. Yet in a herd with the wrong mare, it was a constant battle. Which brings me to another point, I also find the closer they are on the pecking order the more likely there is to be fights. It doesn't matter if they're young, old, dominate or weak willed. Too close on the scale and there are usually problems. Would I put my dominate mini with a dominate big horse?NO! I don't think I'd want to put her with even a dominate mini. Another reason she ended up in with the big gelding was from the fights she was having with an A sized gelding (and oddly enough she was on the loosing end of most of those battles.)
So no I would NOT put a mini where it doesn't work, but with this teddy bear of a gelding who can not go with the other big horses because they run him thru fences and terrorize him, it works for us. I'll take my chances where they are because both those horses will definitely get hurt/or hurt another in the other pastures with more closely sized horses. I don't just randomly stick horses together, I watch them closely for disagreements and move them as needed.
