Integrating 4 week and 3 week chicks

Hi,  I have 4 four week old partridge cochins in a brooder and 2 legbars in another brooder.  What I would like to know if it will be ok to integrate them now or wait until they go outside at approx 6 weeks. 
The concern is obviously the cochins are alot larger than the legbars.  I am concerned about this as I have one female and one male legbar which I will hopefully rehome so dont want to leave it too late before introducing her to others cause not good once she is on her own to integrate her.

Any advice gladly received.

Also when can you start feeding treats to chicks ie, mealworms, apple etc,.

Chickway.