I would plan to have separate housing for the new ones at first. Goats, like chickens, can be brutal to the newcomers and you don't want to just pop the new ones into the same pen with yours. I keep newcomers separated by a fence for a few weeks and then turn them out together in a large area and keep a good eye on them. There will be some butting and pushing and shoving and as long as it doesn't go beyond that they'll be fine eventually.
I agree with not mixing horned and unhorned goats. Mine have all had horns. When they get to rearing up and head butting each other the ones with no horns will take it pretty hard on the head!