Mini horse as a companion

I think donkeys are great companions too, but mini is also a great idea. I know many people who have problems with other horses because their horse is alone all the time.

I bought a mini as a companion for my mare when I moved her home to my property. They got along great from the beginning. I did not have to change any fencing, but even without the horse wire I can't imagine Lily trying to squeeze through the boards...but she is pretty easy going. 
Good point by Horselady to try to be sure the little one doesn't freak out when you leave with Mister. I have the opposite problem; the mini is fine by herself, but my mare would freak out terribly when I'd take Lily out for a walk or a groom.
My mini mows the lawn in the back yard for approx 2 hours a day (not terribly lush grass); she also has free access to Bahia hay, and one square bale lasts about 3 weeks! Mine does get a handful of grain twice a day, but it is really more as an incentive to return to her pen after grazing time is over.
Vaccinate, deworm and trim as any other horse...I trim my own, but I have not heard of many farriers having an issue doing mini feet as long as the pony is well behaved. I do not use quest dewormer with the mini as they seem to be more sensitive to accidental overdosing.
I have also in the past kept my horse in boarding situations where minis were part of the herd and everyone got along great. Many times the minis ruled to roost.
So in short, in my experience minis make great companions, eat very little, and are fun to have around.
In my case the mini is now a solo horse on my little backyard property, and the mare is boarded out again... I am really enjoying the mini as a pet and walking partner (along with the dogs), and Jessie ( the mare) is much happier on 7 acres of grass and multiple friends, where she'll never end up truly alone when another horse leaves for a ride. She has some anxiety issue, but that's another story....

MM, Jessie is boarded with a lady in Loranger. It is a private barn; just Jessie and three other horses on 7 acres with open access to the barn. It's the same place I used to keep her before we bought our property. I feel very comfortable with her there; great care and her rates are very reasonable for this area.
I understand why you feel you want to get a companion for Mister, but it can definitely have both pros and cons. If they don't get along, or get too attached, you have a whole new management issue to deal with...
Lily seems to be fine being the only pony here. She certainly gets more attention now that I don't have to worry about Jessie busting through the fence or digging up muddy trenches in the pasture with her running and pacing every time I take Lily out. Like Mister she will get excited and talk and carry on for a little bit when we encounter other horses on one of our outings, but as soon as the other horse is out of sight she's fine again.
Maybe three is the magic number of horses for a solo rider, so no one gets left behind alone, but I can't keep on collecting horses, lol