Yes, there are differences in the behaviors of jennies and johns/jacks. Like a mare, jennies are more mareish (jenny-ish?). A donkey stallion (jack) behaves much like a horse stallion. Johns (donkey geldings) are usually
just as calm and complacent as horse geldings.
If your gelding is proud cut, I recommend a jenny. If he is aggressive, we'll likely treat a jenny more kindly. If he got after a john, the poor thing would be beaten to a pulp. This is assuming he even recognizes the donkeys as equine. Some horses whom have never seen a donkey ignore them. I've heard a few cases of horses being overly aggressive towards donkeys, but I've never actually seen such a situation.
When placed in a herd with horses, donkeys tend to be very low on the social order and are somewhat picked up. This is no cause for alarm, but I do recommend feeding the donkeys in a separate pen, so they get enough food.
As far as keeping mom and baby together, I see no problem with this. Baby donkeys require the same handling as horse foals, so think twice if you just want them to be pets. You have to train the donkey foals to pick up feet, lead, load into a trailer, etc, just as you would with a horse.
If you have any other questions, feel free to ask!