From what I am reading, you brought in the new barred rock a few weeks ago and she was the first to show symptoms, right? Since then, several more have started showing symptoms, right? There are a number of poultry diseases that can cause respiratory issues and I am far from an expert to know what could be causing your birds to be sick. However, I am thinking that your new girl brought it with her. My best suggestion is to separate all the sick birds form the healthy ones. If you have only a few healthy ones, it may be best to pull them out and keep them in a separate kennel or something since some disease spread through feces (so keeping them in the old area may not keep the protected even if they are away from the sick birds). Unfortunately, some of the diseases are treatable; however, the bird is always a carrier meaning that new birds would be susceptible to the disease. I think visiting a vet would be a good choice to determine what the problem is and how best to treat it.