Yes, I'd think a shetland and a donk would be well matched, diet-wise. They shouldn't need anything else diet-wise, but a good quality complete supp for nutrition is generally a good idea. I agree, as others have said, that while you say your field isn't 'lush', it may well still be a problem for them, as Shetties & donkeys are extremely prone to health probs due to too rich/much feed. You know your own property, but I'd do the research & consider getting grazing muzzles for them, at least for the growing season. You can also restrict their grazing and also encourage more exercise by using electric tape to make 'laneways' & tracks around the paddock. See safergrass.org for one good source on further info.