I don't believe so...at least they're not supposed to. Someone on here was sold a different breed by accident. I personally just bought a Jersey Giant that has grown into an Australorp. It happens when you're hatching out a variety of different birds all at once. Do you have a picture you can post?
I haven't seen any with dark legs either and I got really carried away with sticking eggs in the incubator. All the chicks have yellow legs.

My cockerels are too young for me to make a call on aggressiveness... but from what I've read here and on other websites (ie Greenfire Farms website), they are not usually prone to aggressive behavior... I'm hoping that is true because I would love to keep a few of my roos around - they are just beautiful.