Chicks start fighting from week one onwards, typically. This involves both genders. They rarely hurt one another, as it's mainly play fighting with some semi-serious hierarchy battle-practice mixed in, and any harm done is unusual and generally indicates a future bully, killer, or cannibal. Those are not latent traits in all chickens. Some have them, some don't. (Some cannibals begin cannibalizing and mutilating others before the others have even fully hatched, while they themselves are only hours old, not even yet hungry as they still have two days' worth of absorbed yolk sac to tide them over. Obviously that's utterly abnormal behavior for a chick.)
Generally the fights start getting more serious around the 6 month mark with some more followups around the year old mark as their adult hormones are spiking around those ages and they're gaining size, becoming more intelligent and experienced, and those with a drive to improve their social status will then act on it if they feel healthy enough. Keeping them together from hatching onwards reduces the extremes of violence in the average flock but some are extremely violent by nature and are not content to merely settle the social order, but will pursue the subordinate chook to harm it further even when it is no longer presenting a challenge. Bullies should be culled early or separated as the stress and damage is very counterproductive.
There is no guaranteed age at which they all become violent nor any guarantee of what degree to which they will become violent. There are only generalizations, and since each individual will mature at their own rate, you may be seeing frequent hierarchy reshuffles.
In a normal, peaceful flock they will never harm one another but the genetic basis/inherited tendency towards heightened aggression determines how aggressive they will be to a very large extent so if you buy from someone who has aggression issues in their flock, most likely the birds you get from them will be counterproductive. They are all individuals though so unless you're quite familiar with the social traits of each birds' last few generations of ancestors, probably best to play it by ear. They can, and do, become obsessed with bullying if they are so inclined.
ETA: I'd raise them together with the main flock, always have, for best results and only remove them if they're being harmful. After generations of selecting for calm social characteristics I don't have the 'terrible teens' stages anymore.
Best wishes with them.
