On the how long after planting to allowing grazing question:

You want the roots to be established good enough so that when the horse eats the grass it won't pull the roots up. Too early and you will loose a lot of what looks like really nice grass simply because every time they take a bite they pull the roots out of the ground. You can tell by grabbing some with your hand and seeing if the grass breaks off or you pull the roots up.

That simple.