First goats will eat some thistles but not all varieties - that said my goat field is free of docks, and thistles, the sheep field is not.
However mixing goats and sheep is not advised as both species share the same kind of worms. Adult sheep tend to have a greater tolerance of worms, but goats - being browsers rather than grazers - haven't built that up and suffer quite badly from worms. So if you are planning to mix them, you need to be certain of the worm status of your field.
Goats also need a shelter for wet weather and overnight, they do not stay outside all the time. Goats also need hay all year round, for overnight only in the summer, all day in winter - my goats do not go outside much in winter, as I rest the field from October till March.
Goats are quite different from sheep in many aspects. Do some research - your local library should have quite a few books. Even the older ones from the 80's and 70's are still quite relevant, except for chapters on worms and disease control.