I would strip graze in roughly quarter or half acre sections. As said above you need to monitor intake as they are very prone to laminitis. That way you just need to rotate it when you see the grass has gone down or it has turned to mud (depending on the weather). Minis practically live on fresh air, if you find they're losing weight get a vet out - mines lost condition over the winter despite getting plenty hay, turns out she has Cushings. You don't want rolls of fat either, it may look cute but is really dangerous for mini shetlands. If you do feel the need to give any hard feed alongside the grass, make sure it's feet friendly - HappyHoof is laminitis friendly.