Chicks in with the big girls

We have 2 6 week old bantams who are currently living with their foster mum (pekin) in a separate house and run within the main run.  When and how should we integrate them with the big girls (black rock, columbian blacktail and bluebell) ?  They can see and be seen but have not had any contact.  We're about to start moving them onto growers' pellets.  Also, any idea when we might be able to tell what sex they are (hints welcome).

Many thanks

Karen

Thanks Chick Chick.  That makes sense.  Should "mum" go back with the others sooner?

today we built a run within the big girls run so they can have close contact but the chicks wont get attacked (i know my old girls and how they reacted to new hens earlier this year!)

they will be like that for a good few weeks and during that time we will start to open the gate so they can mix until i am happy they can be together, mine are only 10 weeks and there is no way i could let the big girls near them just yet after the way they treated the 16 week old ones who were a lot bigger. My old girls may just be grumpy though!