Our Sebright is only 1 of 2 bantams in a flock of giant orpingtons & other LF. The bantams do fine & have bigger attitudes than any of our LF birds. Our Sebright is appropriately named "Trouble." She is quick to steal more than her fair share of treats & is very, very vocal if she feels offended &/or ignored. She chooses to sleep above the roost where the top hens sleep, & so feels she's queen of the coop. Our roos are about 10lbs. Because of the size difference, there's no way they could mate her & they'd have to catch her 1st. We were never planning on keeping the little Sebright, but now I can't imagine our flock without her.
I cannot guess how a medium-sized chicken will react to a little hen, but I can say that a Sebright will not quietly allow others to pick on her.