I've had decent luck introducing a young cockerel into a group. Even then, I have had an older cock start a fight. I've also had cocks that were together their entire life start fighting for no obvious reason. And have lost roos to other roos. I would suggest keeping separate if at all possible...
Yes, have purchased birds from them 5-6 times. Customer service is great. Shipping went smoothly. Most birds were healthy and of good quality. Please message me if you want more details about my experience. Thanks!