I think it depends on the horses involved. We took in some rescue minis and never put them with our full-sized horses in part for quarantine reasons and since we were finding them new homes, we didn't need to integrate them with a herd. However, if we were to put them with the herds, I would have not put them in with certain horses simply based on those horses behavior in a herd situation. I think you have a pretty good handle on your horse and how he is, you just have to "feel" when the situation is right. Good luck...
and enjoy those minis!