So I wanted to update and say that althought we keep our mature birds and are chicks separated in the coop, we let them range together all day, and we've had no problem. If anything it's better to have the adults around. The mature hens were a bit nasty at first, just letting them know they were the top hens. But the rooster protect the chicks when they're outside. How awesome are roosters! They make certain noises when they think there's danger and the chicks run in the coop! Haha, it's my first year with chicken so all these things are so awesome to discover.
I also got a d'uccle bantam rooster and he sleeps with the chicks (I didn't want an extra rooster the mess up the rooster-hen ration that seems to be working), and he just loves having chicken his own size around him. He did peck at them a bit, but again, just to make sure he's top rooster and now everyone gets along 
Chicken are so interesting!