What color feet does the baby have? What color feet does the roo have? It may help you some because I noticed your black hen has black feet and if the roo doesnt and the baby does...wouldnt that fit? I dont think its the partridge color one. But, I could see it being the white one, or if the black one is a black sexlink, it would make perfect sense. What other breeds could the black be? Exactly what assortment did you get? Usually they list the possibilities with the assortment. I realize they will never tell you which ones your getting, but this would help us narrow it down.
ETA: your chick looks like a pullet right now to me. I am thinking the reason someone said its a cockerel is if its barred rock, the cockerels normally get a white spot on their head...i think...dont quote me on it.
Thanks for all of your help! I was guessing that Mildred (the white one) was the mom, otherwise Gypsy, Australorp, would not be a purebred as daddy is also Australorp.