Yes!
My first mini was a companion for my then 21 year old mare. Now my mare was not a huge horse, more like a LARGE pony, but my mini was only26.5 inches, AND THE MINI WAS THE ALPHA MARE !now I don't think it would be wise for an honery young full size horse that is aggressive in nature to have a wee little one around, we need to use good judgement.
My horse had lost her companion who was also her mother, after being together for 20 years, she was deperate for company and willing to accept any new friend.
Remember, horses don't nessecarily know their size, minis tend to think they are big animals.
Promise