Chick, did you search the forum for information on cocci and mareks symptoms, just so you can be informed? I know if you see anything in the feces that looks like blood, you should look for treatment, and I also know that birds have several different colors of "normal" colored poop, depending on their diet. Your chickens do not appear overly thin to my inexperienced eye, but there are many qualified BYC-ers here who might be able to look at those photos and give you more peace of mind.
It's going to be a bit cool tonight. I tend to let my ducks and geese go out during the day starting a few hours at a time during the day and see how they do. Then, when it's going to be a nice, clear night, I pick their first night outside. As long as they are doing well, I leave them out after that.
Maybe you should look for a post by Glenda Haywood and PM her?
OK, all out of suggestions!