I will be one to say we do have our pretty robust, 34"/250 lb mini Marty in with a 14ish hand gelding, Oz.
As a back story, we have 2 other mares, Sara - a warmblood who kicked and killed a big horse in turnout many years ago before we owned her - and Josie - a small draft who was born on a Premarin farm and poorly socialized, I think, and basically hates all other horses, so both of those gals have separate turnouts.
Oz and Marty are both 30 and I have never seen Oz really kick at anything. When we brought Marty home we made a separate small pen of stock panels within one of our turnouts, first with an alley of electric fence around it for a few days. We graduated to turning Marty out in a round pen in the middle of Oz's turnout, which worked really well. The other horses would come over to Marty and try to touch noses and he would go absolutely insane, squealing and striking like the black stallion, and they would just look confused and walk away. After about two months of this and never seeing Oz retaliate once I let those two out together and they are fine - I know there isn't zero risk but it feels reasonable. I wouldn't chance it with the two mares just knowing their history and marish-ness. Both the boys need soaked diets, so we separate 3 times a day for feedings and overnight.
Our horses seemed genuinely confused by what the mini horse even was, their faces said "Is that a horse?" so being able to just see him for a few days was helpful.
