We didn't expect them to not be on the bottom. We just put them there because it was getting too cold for the Japanese to be in their own coop in the winter
They were strangers. I got the Japanese last year from a 4Her and after quarantine just stuck then in the coop at night. They were adults and my hens at the time were adult
I can slightly agree about two roosters makingbit harder for peace, but I don't agree with the sizes making the flock incompatible. My flock has small bantams like Japanese hens and then larger standards and they're all just fine.
They might be fine, but I would have a back up just in case. I had one pair of roosters that was fine until I separated them for a week, and another rooster that is fine with his sun's until they're about 7 months and then he gets aggressive at them