Ohhhh...he isnt already gelded? How old is he?? Has he ever been used for breeding? A jack who has been used for breeding, then gelded..could be set in his ways. A breeding jack will usually get very aggressive towards jennys (especially standards) when the jenny comes into heat. Jacks can be very unpredictable, and being a standard he could hurt your jennys, if they are small miniatures. So, you might want to take all of that into consideration BEFORE putting him in with your jennys. Some jacks will "run" a jenny until she lets him breed her. I thought he was already gelded. Also remember that after he is gelded..it could take a long time for him to calm down. Everything is going to depend on the above. Some can be sweethearts after there testerone level drops, others assert their dominance throughout there life, or at least for a long time after being gelded. Without actually knowing the donkey it is going to be really hard to say. Are you familiar with this donkey? Hope this helps you a little bit.
Corinne