What age they can go outside depends on your current weather. Generally speaking, once they are fully feathered, they can live outside comfortably 24/7 provided they have a shelter from wind/rain/snow.
Every integration experience is different, every flock is different. When I integrated chickens, everyone was fine after just a few hours. When I integrated ducks, it took close to a month and was a much slower process.
Start with a cage or a divider where they can see and hear but not touch each other. If everyone seems to be ignoring each other, you can try putting them together while you are there to interrupt any bad behavior. I found a long stick or broom helpful to separate birds or keep back an aggressive bird while I caught others to separate them once again when things weren't working (break up fights or breeding attempts). Try putting them together supervised for an hour. Keep increasing the time until you are comfortable that no one will be seriously injured. I got to a point where most got along and there was one "bully" picking on the new birds and instead of separating the new birds from the old birds I just separated out the bully until things settled down. Again, this could all happen in a day, or over a month, or longer, everyone's flock dynamics are different.