I would quarantine your new boys for about a week or so, so that they can adjust to their new surroundings. You should keep them in a different room and start swapping toys with your older rat so that they can get used to each other's scent. After a week is up, put both cages in your main rat room. For about a night, swap the cages they stay in so they can adjust to the different scents further. After quarantine, it's alright if the cages are beside each other but make sure that can't touch each other. Monitor them closely during playtime, as something could happen. For example, my little Lea had a toenail ripped off by one of the older girls while the older girls were playing. Don't have them playing with each other until they get along and know each other well.