anyone know what breed this chick is?
How did you come by these chickens? Did you purchase them, hatch them or were you given them from someone's hatch? If you purchased them as "pullets" there can still be a margin of error. Up to 10% might be roos. But from your pictures, there is no reason to be alarmed. Keep an eye on the combs, A roos' come is going to grow faster and turn red early. A pullet's comb usually wont turn bright red until they are ready to lay, (around 20 weeks give or take a few).