I think some people have had luck in keeping them alive by rationing their feed and making sure they can't eat as much as they'd like to. You might want to do a search for that on here and see if you can find it. Just keep an eye on it - if it's gobbling down feed and growing much faster than the others it's likely a Cornish cross. They get into the other bins sometimes - just yesterday I had to tell them to remove one from the silkie bin before someone ended up with it on accident.
I'm not sure what the little gray headed chick could be, hopefully someone else will pop in who knows. Also, was the top chick sold as a pullet? I looked at some earlier pictures you posted and it looks like it has two little stripes down its back. Is it yellow as it appears in the picture or is it more reddish in real life? After seeing that other picture I'm wondering if she's a sex link.