Donkeys and ponies should get on fine, once they have been introduced and allowed to get used to the other. It's just that some horses/ponies, which have never known donkeys, can take fright when first put together.
A friend of mine who keeps donkeys took a took two horses on temporary livery, and just put them in the field. The donkeys were in the corner of the field but, when they saw the horses, one started tuning up for a bray, and the other started to walk toward the horses. Both horses took one look at the donkeys and just bolted. One ran round the field; the other ran straight into the gate they had just entered the field by.
Another woman used to keep donkeys, and a livery business started next door. For a while there could be no horses kept in the field adjacent to the donkeys because the horses were afraid to leave the gate and graze toward the donkeys. The horses spent a lot of time throwing their heads up and staring at the donkeys if there was any movement from them. Any horses which were eventually grazed there had to be carefully slected for their acceptance of donkeys.
I knew a man when I was young, who kept lots of ponies and donkeys, but always grazed them seperately. He said not all combinations got on, and it was easier to keep them seperate.
However, it seems that some ponies are happy to live with donkeys. You see it a lot in Ireland. I suppose, as long as there are no bullies in the herd, they may get on fine, once they know each other.