I have giant cochins, bantam cochins, jersey giants, marans, silkies, EE's, and a couple of other breeds together in my laying flock. They get along fine. I have some extra sussex roos that were in with them, but they were harrasing the smaller girls so I seperated them into their own run.
You do need to keep the new birds quarentined for a while though before you put them in with the others.