I got a mini for the same reason - to keep my retired mare company when we took the other two off the property. She is not a terror at all; I got her when she was about 20 and she’s turning 30 this year, so maybe that’s the difference. She was part of a breeding herd dispersal, so I don’t think she was handled much. Once you catch her, she is just the sweetest thing (as long as you don’t give her a vaccine or a bath)…she’s sort of like a Gremlin that way.
My horses were totally fine with her, as if they had all lived with a mini before (and maybe they had). I kept them separated at first while we gauged their reaction, but it was clear they were not afraid of her (or vice versa) and they instantly just added her to the herd.
The bigger issue is that she can’t be turned out on pasture, at all. So in the summer when they went out for night turnout on the pasture, she couldn’t go and she was alone. I ultimately got another small “can’t have grass” metabolic pony to be her companion…and not surprisingly, they don’t like each other. LOL. He wants to be with the big ladies, and she thinks he’s annoying. But they get along well enough.
The turnout issue is big concern. Minis typically can’t have grass (or much of it) so if you have pasture it will be a challenge. Mine have a sacrifice lot they can be in but even that grows too much grass in the summer so they need further accommodation.
So I’d be cautious in considering one, but the evil one in my herd is that damn pony (far right of photo). He is the fence breaker. But I think all small, hairy equines are tough on electric fences - they can slip between/below them and their coats are so thick they don’t even get shocked. And between that and the fact that they can’t tolerate much grass - it can be a difficult problem.