Many people do not know the dangers of founder in our area, unfortunately, and I see MANY minis turned out with big horses, in lush fields, as well. However, just because it is done doesn't mean it is correct. Theres a good chance that those minis HAVE foundered and that the owners just arent educated on it and havent noticed. My own miniature foundered on pasture before we got her, as a weanling, and she is 'ok' thankfully but has had slight coffin bone rotation and has weak hoof structure because of it, as well as tell tale signs of founder in the way her hooves grow. We had another mini who broke into the grain room and ate 6 lbs of senior food, and foundered so severely that even with VERY frequent, specialized hoof trimming and a special diet, he still had a severe limp and had to be put down a few months later.
Shetlands are a littler hardier than miniatures, IMO, and although they should never have 24/7 grazing, I think they're less likely to founder on limited grazing. (a few hours a day in a well grazed pasture or so) They're also a bit bigger but rarely eat more, which might be a plus for you if its going to be with a full sized horse. I'd get a 40"+ one, with no poor health history and good feet. Don't get a stallion, and make sure that it has a negative coggins, if nothing else, and feet that have been trimmed recently. There are a LOT of shady backyard mini and shetland breeders in our area so be careful.
It shouldn't need any grain.