So very much of having successful pasture mates depends on the personality of the animals, not just size. Two neighbors have a mini with big horse and no issues whatsoever. But, they are calm horses, were fenceline introduced, have large pastures to roam, etc. I have a B mare and a tiny 27" mare who are best friends! They eat together, groom, etc. The tall mare was at the bottom of the pecking order in the group her size, so I moved her because of that. The small mare was more assertive in her group than this mare would ever have been but, not mean. The little one kinda took up for her when the other small mares shooed her off. It has worked out fine.
Hot blooded, stallions, ill tempered &/or very dominant are things to be extremely cautious over.
I often look to who is compatibe at the feed trough, who needs same type feed, etc. Have a field of mares who are 20+, the don't fight anymore
So, only non-agressive types in that group, and those who need their type feed, shelter, grass, etc. They still enjoy the foals, so I sometimes put a couple of the younger mares with foals in for some "grandma" time.
Know their personalities, any special needs or concerns, go from there.