Opening the mouth wide can be yawning, although if the chick is doing this all the time it could be an indication of an infection, or parasites. If it only happens every now and then, I'd keep an eye on it and make sure that the chick is eating well and gaining weight. If it's happening all the time, like several times a day for more than a day or two, there could be a problem.
The two little sand-colored chicks could easily be full siblings; one just got the gene for single comb and the other got rose comb. They are at that age where it really is difficult to tell gender. I often find that photos are useful, because when I go back and look at them again, I spot color in combs that I didn't notice at the time.
Feathered legs: lots of possible sources, could be Cochin in there, or Brahma, or Silky, or Belgian Bearded . . . I'm sure there are more, and I've also noticed that feathered legs can pop up when both parents had bare legs. It will be easier to tell breed parentage when they get a little older.
Our main flock is a mixed bag. We started out with a variety of purebreds from the feed store, and have let them cross any which way. Some very interesting colors and shapes have come out of that! Especially when one or both parents were themselves mixes. It's been a crash course in chicken genetics, that's for sure.