I agree with Chanda on the darker lower legs and face. Maybe a dunalino, with possible grey? I can't see his back from the pictures, does he have a stripe along his topline?
As to the supplements question, they're horses. I'd start him on what worked with your biggie. Just do the math to scale it down to the right amount for the minis. If you haven't already got one, a digital kitchen scale is a mini owner's best friend!
How exciting on your two new friends. I'll be looking forward to seeing pictures of your mare.
Oh, p.s....your rescue may require you to sign a "no breeding" contract with your adoption contract. Most reputable rescues highly discourage breeding. Overpopulation and contributing to why horses (or any animals) need rescues.
Edited to correct spelling.