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How many roosters do you have? My experience with roosters says they don't generally like to be around a lot of other roosters because of dominance issues.
When you keep multiple roosters with hens, I think it depends on the attitude of the individual roosters, how many hens you have, and how much space you have. Having more than one rooster for every 8-10 hens or so may lead to some issues: fights between roosters who want to be dominant, roosters becoming more aggressive, feather loss and stress on the hens from being rode too hard (too often, by too many roos). In my opinion, unless you have a lot of space or free range and have plenty of hens (10+ per roo), I wouldn't try having more than one or two roos. I had 3 for a while in an approximate 20'x40' run, but eventually there were issues - lot's of fighting, one rooster in particular got aggressive with people as well, hens were bothered quite a bit. The two roos that remain have gotten along quite well. For the time being, at least, one seems to be content to be 2nd in command.
As far as when to integrate your new birds, you could put the cockerel(s) in with the others when they are nearly mature and can protect themselves/run away if necessary, just like the pullets, around 18 weeks or so should do it.