My RIR's are also mild mannered. Only have one left currently,

but I would not hesitate getting more in future.
I think the key in chicken behavior is the amount of space. When cramped and crowded, some turn to bad behavior, while others become the victims. It is easy to make an assumption that the BAD ones are that way by nature, and the VICTIMS are always the mild mannered chickens. Then fast forward to having just the mild mannered chickens, and a batch of even more docile acting chickens!!!!!!!!!! I think in same environment when crowding is issue, those medium mild chickens would turn aggressive towards the least aggressive. Size of chickens does play a factor. RIR's are quite large.