Not sure if you have tried this but if you type in nutrition in the search topic on the bottom left hand side of your thread you will get LOTS of nutrtition information off of Lil' Beginnings.
i am looking for nutritional % to feed for grain. I have found protien and fiber requirements for a healthy horse/mini. Though I am having a hard time finding % on other vits and minerals. I have a 28" non-breeding stallion that I rescued 3 months ago when he was quite thin. I am used to taking care on full sized horses and have been very concerned about colic with my mini. right now he is getting all he can eat soaked orchard grass(he is not a big drinker) and 1/2 cup purina senior horse am/pm with 1tbls corn oil. He is looking a whole lot better, but I want to get more wieght on him before winter hits. I also want him on a diet were he has all the nutrition he needs. I know he is not getting it now.
I have looked through the nutrition info on this forum but would like to find a nutrition outline for daily needs/values/%
I'm hoping this link can give you some good information.
Best of all is to figure your mini's weight and go from there with a formulated feed. Your vet can also help you determine a good feeding regimen for your particular horse and situation.
It is, traditionally, the same for a small horse, just smaller amounts.