I don't think that you will be happy with keeping them.
The Meat, Etc. forum has discussions on keeping Cornish X's quite often. I remember one BYC'er who posted a picture of her full-grown CX. She looked very well cared for in the photo leaning on a feeder. She certainly fit her name, Snowball, but I seem to recall that she only lived to about 10 months and she was a sole-survivor by then.
My brother kept 7 CX's and gave them to me 35 years ago. This hybrid has probably come along way since the mid-70's. I kept them for several months, they didn't do bad laying but I gave 'em back. These days, I think the CX has evolved into some kind of "nuclear android" chicken.
If you want to have meaties and layers, buy extra Australorp or Orpington pullets and keep some, process the others. If you want to breed hybrids, that's possible too. The Plymouth Rock is a fine bird and a parent of the strains that lead to the CX. A flock of Barred Plymouth Rock hens has the advantage of hatching sex-linked chicks when crossed with something like a Rhode Island Red (or Buff Orpington, so I'm told) rooster.
Steve