Ideally, you'd want to wait until all of the birds are roughly the same size. That being said, its difficult for most people to do that due to lack of space or separate enclosures. They can go out to the coop when they're fully feathered. Or maybe sooner, depending on the temps in your area. However, I'd recommend keeping them sectioned off in the coop or run (safely) for a while so the bigger birds get used to them being around. If you just put them out, you can expect the adults to treat them very unkindly. They may only give them a few pecks to put them in their place, or they may bully them relentlessly. The chicks will be at a serious disadvantage due to the difference in size if they're still small.
Our new additions are coming in on June 16th so we will probably keep them inside for a few weeks under a heat lamp until they get a bit bigger. I am building a new coop that will have a removable section that I can raise chicks in. The dividing wall in the coop will be removable so when they are fully feathered I can remove the division in the coop and they'll be together. Once I get it build I will post some pictures.