You have received some good advice here and obviously situations when it worked. However - and this is a big however, most serious Mini owner/breeders, do not suggest putting them with full size horses. One kick, even playfully, from a large horse, can kill a Mini. A pushy Mini can upset a large horse. This can spell disaster and has been well documented, many times over.
If you must keep a Mini with large horses, make absolutely sure they know each other well before placing them together. We suggest though, it is better to keep the littles with littles and the bigs with bigs.