My first flock of chickens I had all sizes together and didnt have a problem at all. I do have a really large coop, so maybe that was a factor. I did not have any issues, that is my experience.
Has anyone ever had two hatches going on in the same incubator? Is that possible?
ie. The first set of eggs werent developing, only a few were, so you keep the ones that are, throw out the bad. And set a new batch in the same incubator.