I have a standard size donkey. I have a few books on the subject, all my knowledge previously was on horses only, and follow Lucky Three Ranch feeding guidelines. You would split these measurement between the two Mini's. Winter time: I give 1/4 Cup per day increseing every other day until we reach 1 cup (by weight) of oats, use rolled or crimped, 1oz of sho-glo-1 oz of corn oil, 1 flake of hay at night. Spring after last frost: take a week or so to lower oats by 1/4 cup a day or 2-down to a small handful-just enough to mix the sho-glo and corn oil into, if you have grass by now then stop the hay. There are certain hay to stay away from-cereal's does NOT do a body good if your a donkey and oats will mostly pass straight thru if not crimped or rolled. The sho-glo and corn oil are for hoof and coat care, and to keep the intestines regular. Mine must wear a grazing muzzle 23/7. Once a day I add a lead line, remove the muzzle, feed the oat mixture, re-muzzle. I would prefer to only use the muzzle during the day--but He will not let you catch him to put it back on!! You can also keep them in a dry lot over the summer. Donkeys in the wild, are from very arid climates,and usually spent the entire day walking all over to find enough to eat. So, they eat far too much in a normal pasture situation. My farrier also keeps a close eye on his weight--I don't know if we can ever remove the fat crest from his neck, but it is much better than when we first bought him. Mine also gets wormed every other month.
Did you know donkeys are second only to camels in going so long between watering's? that said, it's important to have a supply of fresh water and to add a heater in winter because the water can get cold to the point they will not drink enough, mine's on a thermocube so it only comes on just before freezing. I don't think it gets freezing where you are in LA.
Your former owner is correct donkeys are on the greedy side, but a flake a day of hay shouldn't hurt between two --just watch and feel through the coat for signs that they are getting overweight.