It is always a good idea to carefully quarantine any new
rats for *at least* 2 weeks before introducing them to your existing rats. This should be done regardless of where the new rats are coming from-- including breeders and rat shows. Even rats that look healthy when you bring them home can be carrying the vestiges of an illness that may be contagious to your rats.
articles should give you some tips on safely quarantining rats:
Quarantine: Protection for Your Pets
Facing Deadly Viruses