I am always getting new chicks,so far every year. I recently got six more.Thought I might share how I introduce mine to the flock. I get them and let them relax.Always let them adapt to their area first.After having them for about a week,I let them grow some feathers.I usually start taking them outside by two weeks old.The first thing I do put a hen or two in with.Pullets can also be aggressive, and there is no way of telling gender by how dominant one is. I know my chickens very well.Some hens no how to handle chicks, while my year old hens don't very well. Some chicks are brave and are more dominate then others.This being said,some chickens are more dominant then others.This just means they like evrybody knowing they aren't afraid their the boss. Lot's of times young cockerals are dominant up until they find out their to small to fight the bigger hens or roosters. Something I have observed is the lowerbin the pecking order a birds in,they will be announced towards the chicks.I observed my bantam with chicks ,(keep in mind shes lowest on the totem pole),she was very dominant with the chicks, however the more the chicks backed down (yes they were trying to fight her),the friendlier she became,in fact she even played with them.The only reason she was out for blood at first was because she most definitely doesn't what the chicks disrespting her,and she doesnt wanna be put at the bottom any longer.After I put her in,I had put another low ordered hen in,and she was out for blood as well,in fact she didnt stop (This is your que to seperate them).So I took her out and had them in with more dominant hens,they were nit nearly as aggressive, in fact,some of them acted as if the chicks almost didn't even exist.Then I put my head hen un,and she didn't do a thing. I do this up until I put the chicks in the actual run&coop.Everything works iut.Also try to add treats in while the chickens get to know everyone.