My brother kept a meat bird with his free range layers and it was healthy until a predator got it. I have the same issue with my first little 6 bird flock - was sold a Cornish X but told it was a Buff Orpington. He told me to just not free feed her with the test of the birds, and let her forage all day, and feed her before bed for no more than 15 minutes so she goes to sleep with a full crop, but is motivated to find food all day and hunts for it. It will be an added layer of complexity to caring for them all having to treat one differently when it comes to food, to allow the layers free access to their feed but somehow keep the Cornish from getting to it, and making sure she's got plenty of access to pasture and bugs. What I think will be even more difficult for me is knowing if/when her quality of life warrants a compassionate culling, like any other family pet.