I have both full-sized and Bantam Cochins, who spend their days loose with a mix of other birds -- including ducks and geese.
My two older Bantams had lived in a mixed breed flock before they came to live with me. Once they settled in, they had distinct personalities with the barred being more dominant, but neither is a shrinking violet.
They are free with the mixed full-sized flock during the day, and they used to coop with a big BLRW. They only sleep in their own coop at night because chicken math got the best of me and space limitations made more sense to house them by themselves. Frizzle Gabby will stand her ground with the big girls, but little Sally will CHARGE the bigger birds if a treat is the reward.
This spring, I got three full-sized Cochins who have no problem mixing with everybody; they stick together most of the time and will dash into the melee for treats but are not aggressive in the least.
Three 4-month-old Bantam Cochins are still in the teen brooder with their flock mates -- a Columbian Wyandotte and a Bielefelder. Two of those newbies are little roos, and they posture with each other -- and through the fencing at full-sized hens.
Just my experiences with Cochins, which I absolutely LOVE!