We keep our minis (two donkeys and a mule) with our regular sized horses. The donkeys generally stay off on their own, but will graze with the herd; the mule gets in there, beats up the yearling and the 3-year-old and then hides under the "big boys".
His best friends are/were Bruce and Randy, who are/were 16'3 and 17hh respectively. Randy got sold today, hence the partial past-tense.
We've not had any problems, although I know it isn't always the case. We used to have a standard (on the large side of that, too) donkey who was GREAT with everything, except miniatures; he went after them with blood-craving passion. It was so bad (him trying to tear down fences to get to the minis) that we had to sell him.