I had a RIR raised by home-schoolers that I bought at 5 months old. She would hover over 2 of her Black Star sisters (that were actually bigger than her) as if she were their mother if there was danger. She was the best chicken of my 2010 flock.and the RIR's have a reputation for being aggressive with the other hens. I was complaining about my andalusian going broody. But, a couple of the other birds in the flock sneezed, and she's been through so much, she gets to stay. So maybe you could find a mellow bird that should tend to go broody, maybe a smaller NN or Dorking?