Do not put new chicks in with full grown hens they will be killed. Broody hens will sometimes accept new chicks but they may not. Chicks of different ages/breeds can be mixed with caution and supervision of the transition period.I would be careful putting EE's and Polish with larger breeds like Sussex, Wyandottes and SL's but it could work out if they are all young enough to bond.
Smart advice! Always exercise caution and common sense with introductions.