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!
Just to note - I have hatched Niederrheiner chicks (shipped eggs) at home. The chicks were also surprisingly small. Not many hatched, but that could be the shipped eggs. Got me wondering if some of these breeds are rare because they just aren't thrifty or easy to grow. The few that hatched were...