Im staring up my backyard flock today and i had a questin i have two barred rocks that are 8 months old if a get a 7 week old Ameraucana are they going to beat her up?
Yes. Introduce them by keeping them separate, but visible to each other (i.e. two different enclosures). Let them get used to each other's presence for several days before subjecting the new girl to getting pecked at. Also, only introduce chickens of the same size to each other. Smaller (younger) ones will always get picked on.
Almost certainly yes. Usually the older chickens will not accept another chicken unless it's the same size as they are. Once in a while I've heard where people 'lucked out' and it worked for them to introduce a much younger chicken into their flock but that's not the norm. So, when people add their young chickens to their existing flock they wait until they are all the same size to do so. If you decide to try it you need to keep a very close eye on them for days and make sure the little one isn't being pecked to death. If it is pecked you need to be prepared ahead of time to have a separate area for it to be safely moved to. That also means (IMO) that you need to have a friend for that chicken so it's not all by itself. Unless, the separate area is next to the other chickens so they can see each other. There will be some pecking when they are introduced, that's the way it works, just watch for injurious pecking.