Sounds to me like you will need a brooder for them as babies, but a coop, or some LARGE area for them to finish out in, up to a month old. Don't they grow pretty fast, and will be pretty large in a month? I would suggest an outside coop, with a heat bulb, if you need warmth for them. But I don't know much about the Cornish cross, except they are fast growers. I hatched 40 Rhode Island Reds, and thought I was in pretty good shape for brooders, (in our heated garage) as I had 3 brooders each about 3 ft by 4 ft, and 2 cages about 2 ft by 3 ft, (Total about 48 sq. ft) and I got in trouble having room for them all, as they grew. I know this isn't much help, but some with more experience hopefully will reply.