In my personal experiences I would not add eggs to an existing hatch, for the reason that depending on your climate or hatching methods you will probably try to increase humidity during last three days of the hatch which creates the possibility of the environment being too humid for the eggs that aren't near the end of their 21 days. Raising chick's together shouldn't be an issue if your brooder box is big enough. Not only room for the extra chicks but also so the older birds can escape the warmer areas if need be. I find the younger the flock starts together the better they are in the future. For example I had ducks and hens together from day olds, however I would question introducing a white pekin to an existing adult flock of hens. I'm sure people have had different experiences however those are mine. Hope that helps!