I had concerns like yours before I got 20 Cornish Rock X's this spring. Here is what I have learned:
If you are in a cool climate, don't get them too early in the year. They will do fine in a brooder, but their input/output is amazing, and the brooder will get stinky very quickly. I had to wait until mine were nearly five weeks old before allowing them out. Luckily my 'brooder' area was very large. I would like to have allowed them out earlier, but it was too cold. At about 5 weeks and a few days:
I have a hot wire high and low around my tractors, and this seems to work. I don't freak out so much when a dog visits. DOn't let it touch the chicken wire on the windiest days. The tractors for this breed just have fence wire strung every 4 inches on top as flight out isn't going to be a problem. My other bird tractors have chicken wire tops.
Listen to the advice of people who say to restrict their feed, particularly if you are using a >20% protein feed.
Allow more feeder space per bird. They are broad, and at about 2-3 weeks, where 25 orpingtons could feed, maybe 10 CRC's would fit.
When I restricted their feed a bit (even at 20% protein), they behaved much more like regular chickens - curious, interacting and have recently begun scratching outside. I would have liked to have 'tractored' them earlier, and will with the next group.
Their growth rate is amazing. I will definitely be including them for homestead meat production.