Stocking rates is always a guideline but 5 ewes to the acre is what's usually quoted for medoum sized ewes on good grass. The total area is best divided up and stock rotated round it. I try to do a month and move, over the summer, then let them stay on one paddock during the winter with hay supplement, lick and a bit of hard feed at tupping then in the run up to lambing. We have pretty free draining soil, so poaching isn't too much of an issue.
I graze adult sheep with my ponies and they're all fine but I have heard stories of ponies killing sheep, especially lambs.
The best book for new sheep keepers is "The Sheep Book for Smallholders" by Tim Tyne. There's also some articles on the TAS website - click on the LIVESTOCK buton at the top of the page.