I had a couple mini stallions with my full sized buckskin mare and they never messed with her at all. More like friends than stud/mare in there together. If she was in heat, they would smell her pee, but wouldn't even try to sniff her. Put the little guys in with a mare their size, they'll hound her to death until she is ready to breed or kicks the crap out of them!
I always left them in together and put the feed buckets higher than the mini's can reach, so there was no way the minis can reach the big horses feed. I also fed them a cup of my regular horse's feed and they always maintained good health.
I never really got why people wanted to keep them separate, other than perhaps one horse picking on another, but I never had to worry about that, lol.