So, introductions with chickens are best done with 1-3 weeks in a "look can't touch" situation. You want the younger ones either behind a fence that they can be seen still, or in a fence that they can get through that the bigger ones can't. Once the older birds have seen them for a while they accept them as part of the flock, and not as intruders. There will be some hazing as the pecking order gets established, but after that it should be peaceful again.
They also need a lot of space during introductions, so the little ones can get away, and the older ones don't feel like their space is so limited that new chickens are threatening it.
You can usually determine sex at around 6 weeks by the size of the comb. At 16 weeks you can often tell because they will be crowing or laying (some breeds at least) at that point.
If you got straight run chicks and you only have one rooster you should kiss your lucky boots. Most folks that get straight run end up with about 75% cockerels, I'd say.