Your coop holds 12 according to what? The humane living conditions guideline says 4 square feet of coop floor area per bird, but a lot of people pack their coops much fuller than that, causing behavioral/bullying issues. If your coop can hold 12 birds, then it must be at least 6x8 feet in size, which is not a small coop and you should have plenty of room to add another roost. If things are so packed that you don't have room for another roost though, then maybe your coop isn't as big as it should be. When introducing new birds into a flock, there's increased tensions and drama, and space is even more important. Consider reevaluating your birds-to-area ratio, especially if you'll be adding new birds (6 is not a small number!), which will rock the boat even more and you'll end up with more problems.
Newcomers are automatically bottom of the pecking order. By putting them on the roost first, you are offending the main flock, whose reaction is to rebel and kick the newcomers off the roost. Top chickens pick roosting spots first, so placing a bottom chicken up there first breaks the protocol. So putting the newcomers on the roost last makes more sense. It would be best if you could have a separate roost for the new birds. Doesn't have to be lower than the other one, just separate - in a different part of the coop, if you have the room, just make it a separate branch/board not attached or adjacent to the main roost. The older birds will likely stay where they're used to - on the old roost - but the new ones will have a roost of their own, where the others can't get to them. But seriously though, reevaluate the size of your coop and your roosting arrangement before adding any more birds. 17 is a lot of chickens and they will need a large space.
Newcomers are automatically bottom of the pecking order. By putting them on the roost first, you are offending the main flock, whose reaction is to rebel and kick the newcomers off the roost. Top chickens pick roosting spots first, so placing a bottom chicken up there first breaks the protocol. So putting the newcomers on the roost last makes more sense. It would be best if you could have a separate roost for the new birds. Doesn't have to be lower than the other one, just separate - in a different part of the coop, if you have the room, just make it a separate branch/board not attached or adjacent to the main roost. The older birds will likely stay where they're used to - on the old roost - but the new ones will have a roost of their own, where the others can't get to them. But seriously though, reevaluate the size of your coop and your roosting arrangement before adding any more birds. 17 is a lot of chickens and they will need a large space.
