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I would definitely take them out together but keep them separated so they can see one another and interact without harm. Then, after a few weeks time, blend them at night by setting them on a perch together after dark. (If you bring the older chicks out first, they'll establish the coop as their territory, making it harder for the little ones to successfully join them later). And, yes, the little ones will definitely need heat.
Keeping them separated inside the coop for a few weeks time may seem like a pain but it's well worth it. The older chicks could easily peck the little ones to death. The slower you go, the safer they'll be. Watch them carefully!
I'm nervous about them going out that young but I tend to be conservative. Best of luck to you!