Like people have been saying, it depends greatly on the horse's personality.
I used to board one of my minis with an old appy gelding. You have to gradually get them used to each other, because a lot of horses FREAK when they see another equine of such small size. A good idea would be to have a "trial run" with your potential mini, to see if the two get along. It would be a shame for you to buy another horse, just for them to not get along and you having to seperate them...or worse.
Actually, I just made a post today on my blog, The Miniature Horse Network, about if minis are right for you. It may help you out. HERE
is the link.
Hope that helps,