Minis require the same as a full size horse, just in smaller amounts. If you need to feed a commerical feed stay with lower carb and lower sugar feeds. Sweet feed is not the best choice for minis as they usually get huge on it and hyperactive. If the mini is fat, make sure they have weight all the way around their body not just a huge belly and no topline as this is typically a protein and vitamin/mineral deficiency.
I know lots of people who keep their big horses and minis together, just always remember, they may be the best of friends but one misplaced playful kick may not bother a big horse, but can kill a mini.
Before you turn a mini out with your big horse, make sure your big horse is not afraid of it, as most big horses who have never seen them before are terrified of them.
You know how to make a miniature horse even smaller? Leave them in the dryer a little longer!
"Don't ever regret something that once made you smile"