You are going to get answers all over the place, but the reality is they should be fed a consistent diet, don't suddenly change it up on them... A goat that has been fed a diet of grain can handle it fine, on the other hand if you take a grass fed goat and give it lots of grain chances are you are going to have issues... Don's suddenly change up their diets and they should be fine...
When I got my goats, I did a ton of readying on preferred diets and to be blunt there is no real consensus...
What I did find is a bunch of people proclaiming, if you feed them this or that you will kill them, followed by someone saying that is the primary diet for my goats and they are not dead...
Mine pasture graze all summer, I heavily over seeded my pasture with a premium horse pasture seed this spring (best $70 I have every spent on food), let it grow up to about a foot tall before I let the goats and llamas out of the barn, and I have not supplemented at all this summer and they are all a healthy weight... For winter I feed mine whatever local 'horse' grade hay I can get cheap that is about a 20% alfalfa, 80% mixed grass, the llamas get a daily supplement of a little grain during the winter and the goats get a treat of grain, not a daily thing... My animals are locked in a barn all winter and way form the elements (not heated but still not wind or snow) so if your are outside a little more grain might help them keep warm...
As for how much, well I found if I overfeed them it just becomes bedding, they just pull it out of the hay rack and waste it... So it became a trial and error process that was pretty easy, I justed watched how much waste was on the ground each day and adjusted until that amount was minimal...