All our animals share the grazing at some time or another. Sheep, cattle and horses, geese too, hens and ducks.
Sheep will eat the bits horses won't, cattle and sheep will eat the worm eggs and larvea that affect horses but don't affect them, thus making the worm burden on the pasture less. Horses eat the cattle and sheep larvae. There is one type of worm that affects sheep, cattle and horses but it is killed by most wormers.
Mixed graxing is a good idea, but don't stock sheep too highly or the poor horse won't get a look in at the grass! Sheep are good at grazing young ragwort down, they can tollerate young leaves and will eat it and it won't get a chance to go to seed.
As for re planting sheep grazed grass, now way, otherwise the whole of Shetland would have to be re planted every year!