You will need to mix the new chicks in slowly as you can't mix them in directly with out a lot of chaos and blood shed. So the slow intro will work also as a quarantine. Once the chicks have grown to 6 or 7 weeks of age, put them on the other side of your pen and section it off, existing flock on one side, new birds on the other, so everybody sees, but nobody can touch. You can use hardware cloth to separate. Leave them this way for about 3 weeks. This will allow for some of the pecking order to be worked out without any contact. You can mix them in after 3+ weeks. Make sure to not cram your birds together, give them enough space and add more food and water stations. If there is too much competition, birds will go hungry and dehydrate. The number one cause of egg binding comes from dehydration because they are afraid to drink or are being run off.