theres alot of factors to consider, but my own rule of thumb is once they pretty much cant be contained in a brooder without a wire top theyre about ready, i give them another week to fill in a few more feathers if the weather is mild .. if its freezing cold you might want to wait longer, but doubtful you'll go much more than a couple of weeks more they'll be getting messy and a constant chore .. and if you have mature birds thats another factor you have to consider, i'd still put them out for the aformentioned reasons, but they will likely need to be seperated at least until all the peach fuzz is gone lol ..im sure every flock will be different in how theyre going to receive the new guys .. a rooster may actually help .. roosters can help you with alot of things in controlling your girls, if you have 'him' under control lol .. not an easy task .. it takes training and conditioning from a chick to get one thats worth a dam for ya, then, if he thinks its beneficial to his food supply for example .. he may tell the girls what you need him to about this or that .. that kind of understanding has to be gently built though ..