For what its worth, I've noticed a couple of things about raising CX compared to other chicks -- even slower growing broiler chicks like Red Rangers -- that worried me at first.
First, they sneeze. I don't know why, but they do. This occurred in a group that was raised outside, as well as groups raised inside using different bedding material (wood pellets, pine shavings straw). I currently have 2, 6 week CX living in a coop with 8 other 6 week old chicks and 5 mature birds. The CX both sneeze occasionally. No one else does.
Second, starting around week 4 or 5, they seem to shed a lot of intestinal lining, noticeably more for than other chicks that age. I've seen this with every batch I've raised. At first, this really worried me, but since all my chicks continued to thrive and gain weight normally, I've stopped worrying. I figure it has something to do with how very fast they are growing.
To answer your original question, losing a couple of chicks in the first few days after they've shipped is somewhat normally, particularly if they were more than 2 days in transit. After that, if your set up good, you should see very few losses, but they still happen. Over the years, I have had one runty, poorly thriving chick, kind of like what you've described, and one with an impacted crop, that didn't make it. If all the other chicks seem healthy and active, I wouldn't worry too much.
I think your plan to give them more space and get them outside as soon as the weather cooperates is good. More space generally means healthier and happier birds.