While I understand you are trying to find a practical companion, animals are individuals. There is no guarantee on how much feed any equine will need/tolerate.
I have 2 donkeys and I love them to pieces. I have had really bad experiences with minis as companions (scaring the horses, being kicked and injured, health issues). Both tend to be extremely easy keepers, which can sometimes be more complicated to manage than a harder keeper, as you can’t just throw them out on lush pasture summer without founder risks.
My donkeys each each get a handful of ration balancer 2x a day. They are out on grass pasture wearing grazing muzzles. They get a thin flake of orchard grass hay (maybe 1-2lbs) in their stalls at feeding time to nosh on while waiting for the big horses to finish. One of the donkeys is new to me and we haven’t gone through a winter yet, but my other usually eats maybe 10lbs of grass hay/day and stays too plump. I feed it free choice for the whole herd so it’s hard to say exactly. Ideally, the donkeys wouldn’t be getting free choice hay… but they are companions for hard keeping TBs.
The donkeys get all the same care as the horses in terms of vaccines, farrier, dentist, parasite control, etc.