Unfortunately, I have had some founder issues, but they were not started due to grass; one horse got sick, one got contaminated feed, and one was due to very poor farrier work. All are currently on limited grass. Actually, almost all my minis get limited grass to some extent. [Two are on drylot, so that's why I said almost; the mare is too chubby to be on grass and the stallion is with her to cover her (otherwise, he'd be on grass).] I'm at home, so can rearrange the horses as I need to all day. Three broodmares with foals at side get pasture time from 9am to 7pm; 1 open mare is with them all day; 2 chubby mares go out from 9am to 1pm; 2 geldings go out from 9am to 1pm (one is chubby, one is Cushings); 1 thin mare goes out all day; and, one stallion goes out all day.
Weanlings and yearlings are not highly susceptible to founder/laminitis, but it could probably happen under the right circumstances.