If your horse runs in the pasture, I would avoid turning him out in grass with the sliders. Particularly in the mornings when there is dew on the grass or whatever. My horse wore sliders, and got around on them pretty well, but they have no traction so if he runs around, like my horse always does, he could pretty easily slip in the grass and injure himself. As far as danger to Rosie... I think the sliders would do no more damage than any other shoe, but if you can, I would let them meet on opposite sides of the fence until you can see how well they get along.