While it's true that many do breed back on foal heat, it is something I NEVER have done. The part about them not breeding back is a myth. The foal heat is "natures way" of them cleaning out the uterus after having the foal. I have bred back on the next heat or the following heat but I only let my mares have 2 foals then they got the year off. The only time I have ever had "difficulty" getting a mare to take after having a year off is if they are a bit older. I always let them come into heat naturally (no shots to bring them in) and if they are older and don't take, that's that is "natures way" of telling me that they don't need to have any more foals. My girls were never baby factories. I still own all of my retired mares (that are still alive) I also won't sell them after they get too old to have a foal. To me, they deserve retirement. To some, that my not seem to be very "good business" but it is the way I do business.
I can't honestly say with first hand experience that you can/should breed much larger stallions to smaller dams. I always did it the opposite. If anything, the stallion was smaller than the mare. I do have one QH/pony cross that I bred my stallion to, but she is aprox 13.3 to 14 hh and my stallion is 15.1h. Not that big of a difference. I personally would be very concerned with an appy/mini cross and I definitely would ask my vet.
BTW my daughter has a bachelors of Equine Science and is working on getting into vet school, and I am a nurse, so I do have some background to what I am saying.