To the OP, you've received some GREAT tips from others. The only thing I can add is to emphasize @aart 's comment on same-age cockerels. Our first go-round with chooks saw us ordering 6 female day-olds. We ended up with 2 of 6 being cockerels. We let them both grow up with the girls, eventually adding 3 more pullets from a local farmer. The two cockerels were fairly peaceable with each other once they sorted out the hierarchy. There was definite harassment by the alpha to reinforce the status quo but it never turned vicious so we let the beta take his licks. He eventually got smart and gave the alpha a VERY wide berth to minimize lessons.
Both cockerels treated the girls well from a food-calling, predator-lookout perspective but the mating was hard on the girls. There was VERY little dancing/wooing from either Alpha or Beta. Beta snuck in quickies anytime he thought Alpha wasn't looking often taking any/every opportunity to jump on the nearest pullet out of Alpha's watchful gaze. Alpha would often catch him in the act, interrupt and then chase Beta off...and then chase the pullet until she finally relented and submitted to Alpha's affections. It was a less than happy dynamic. When Alpha started human-stalking (never a direct confrontation, always from behind and then he'd run away when we turned to face him) I'd had enough and introduced both cockerels to the stock pot...but not until we'd put 7 eggs in the incubator.
We hatched 3 pullet and 3 cockerels out of the 7 eggs. We kept one cockerel. Son-of-Alpha and his three hatch-mates had their own clique for about six months. The original old battleaxe hens took every opportunity to thump manners into his young skull anytime he even looked at them crosswise. He pretty much stayed tight with his three hatch-mates and as he matured, started forcing himself on them before they were really ready. After enough time of playing "top cock" in his own little harem he tried forcing the issue with some of the older ladies. That went very badly for him and after all his feathers grew back in he had matured into a very well-mannered young man having learned from the business-end of very cantankerous old biddies how to properly bust a move, drop wing-tips and dance like a fool to win the ladies permission. Eventually, as he took over the roosterly duties of the flock, the clique-iness subsided and they were all now one flock with him at the lead.
I am thankful for the luck of the genetic-lottery that he is fertile, human-wary but not human-aggressive and very even-keeled as compared to his daddy. I am thankful for the cantankerous old biddies for teaching him to rooster the right way while he was still young and impressionable.
To galefrances, most folks recommend introduction based on like-size versus like-age. As long as they are of like-size, they have a pretty good chance of fending for themselves in the brutal "negotiations" of the pecking order. That said, there is nothing wrong with introducing smaller/younger chooks to an existing flock if one has taken precautions to provide enough hidey-holes/escape routes/safe areas for the wee bairns to retreat to if things get out of hand. Observation is key.
As chickens are VERY social critters, clique-iness is common in merged flocks. It is normal and could possibly disappear as time goes on and they forget that they haven't always been together.