Never kept more than one horse at a time; new horse and donkeys coming
I've kept my own horse on my property for about 3 years now, and have at least two donkeys coming to join the farm, probably another horse as well. The donkeys are easy-going, fairly blank slates that don't have much an opinion about people, I've already met them and they're a hilarious, sweet bunch. The horse in question is a young mare, 6yo, and is quite flabby. She doesn't have many miles on her, but for the very short sessions of riding or exercising she's had, she shows a laid-back personality. I have been told, though, that she acts up tracking one side (I forget which) and that doesn't bother me.
My horse of now is a 12yo dominant mare, not aggressive but simply assertive toward other horses and confident by herself. When my mare meets other mares, she tends to sniff for a minute, then squeal and stomp. With bossy mares, she lifts a foreleg high and strikes the air, bashing the other mare's head with her jawbone. I've never seen her bite or kick, even after a gelding snuck a bite on her rump once, but I'm wary about how to introduce her to a pasture-mate. She certainly needs a buddy, I've tried other animals, I haven't tried sheep, cows, or horses yet.
I could keep the mares separate, as I have two available pastures, two paddocks, and four stalls, but the mares would still be in such a proximity that if they hated each other, they could probably get a few licks in.
My question being, how do you introduce mares with the best influence to a good relationship? What should I expect? What are acceptable behaviors when they meet? My mare is a 15.2hh Paint, the incoming mare is a 15hh Quarab.