Being attacked is the LEAST of your worries. Be far more concerned with introducing disease into your existing flock! You need to keep any new birds quarrentined for ay least 30 days. And quarrentined means TOTALLY apart,so they cannot transmit any virus through the air even. Plus you need to completely disinfect yourself and your clothes before going from one flock to another! If this seems like a lot of trouble,go to the disease/illness/emergencies forum here on Backyard Chickens and read all the posts from the folks that did not follow this and lost their flock or can never sell/swap/trade chickens from their flock due to being carriers of some diseases. Then make your decisions after careful research.
I don't mean to scare you or turn you away from having a flock,just want your experience with chickens to fe fun and not turn into a nightmare!