If keeping them in separate chicken coops, move the chicken coops /runs next to each other so they get used to each other for a few days before putting them all to bed together one night in the hen house. There will be some fighting but the more space they have the better. They should settle down in a few days.
If you collect you birds during the day then put in a separate run (or a chicken coop/house in the main run) also don't forget to give them food and water. Then put the new birds in the hen house with the others at bedtime.
You can add apple cider vinegar to the drinking water. Keep your new birds in the hen house and run for a week or so, so they acclimatize before letting them out to free range if required.