Mixed breeds just means you don't know what kind of chicken you'll be getting and the people at TSC can't tell which chick is which and neither can I so this is why I'm going to a local breeder.
Local breeders are always best. I am actually getting some special eggs from a lady that has bred her Buff Orpington with a silky. I cant wait to see what they will look like. I told her I would be happy to try it. However I know both parents are pure rather then a mix.
2 years ago at tractor supply I got 6 GLW's. I still have one of the older hens now. Her eggs are white though and she is about the size of a bantam so something tells me that they mix their breeds. She sure isn't a pure bred.