Originally Posted by Endiku
Deslumbrar, that very rarely happens. The owner of said horses was most likely ignorant, as any well-versed horseman would only allow the horses to meet under supervision, so that if any agression was shown, the horses could be isolated. It is also extremely unsafe to turn a stallion (of any stature) out with other horses, ESPECIALLY with a mare in question. The full sized horse most likely felt that he needed to protect the mare, as did the stud, and they quarreled.
We have pastured six full sized horses and eight miniature horses together for over ten years with only occasional bickering (concluding in small cuts, bruises, etc) I know many people who pasture minis and horses together.
As long as OP supervises the first few encounters and checks routinely for scratches, kicks, or bites, and since she knows the QH is alright with minis from experience, I'm fairly sure that she will not have problems.
The stallion was a winning stallion - I don't think the people that owned him were really that ignorant AND it was explained that the horses had been pasture mates for a long while, the mare was separate from the stallion.