Go to your extension office and find a guide for the poisonous plants in your area. Identify if you have any of those in the pasture. At this point, it's nearly impossible to control them short of hand pulling but you will identify the problem areas for next year to spray in the early spring. When there is adequate forage available, typically horses will pass on anything that might be poisonous so I wouldn't be too worried even if you do find something.
As far as transitioning them to pasture, at this time of year, I would just wait for a good hard freeze. Wait for the grass to die back than toss them out. It's nothing more than grass hay at that point. I wouldn't worry about grazing muzzles. They all will put on some extra weight and probably get a little chubby, but it's winter. A little extra weight never hurt any horse going into winter.