As far as minis and foals "of legal age" going to slaughter at slaughter plants-- sure, they can, and some places/with some dealers probably do.... but the more professional well established horse dealers sending big loads of horses to the slaughter plants avoid them if possible. That is because the plants -- the ones in the USA previously, and now the ones the same company runs in Canada and Mexico (the European owned plant closer to the US/Mexico border) don't want minis or foals when there are plenty of big adult horses. It takes just as much time and effort to process a foal or pony as it does a full sized/adult horse, but the yeild in pounds is much poorer.
I was told by a dealer who used to ship regularly that his loads were tallied by the pound and by the head, and if his load contained undersized/baby/skinny horses that brought the ratio down below a certain standard, he got docked for it. So he wouldn't take a pony or a skinny horse even if it were free, as long as he could get big fat horses for a devent price, because the small/skinny ones could end up costing him money off the whole load.
Miniature horses and ponies are slaughtered. Google for videos of such and you will find several videos done by investigative reporters covering the horse slaughter industry .Sad but true .... not even mini's are exempt from slaughter.
Here is South Florida, we have a problem of horses being stolen for slaughter. Not being sold to dealers for slaughter...but "backyard slaughter" operations. There is a big market for the meat here. I worry because minis would be so easy to steal....just toss them on the back of a truck.