I don't have any personal experience with sororities, so maybe someone else can chime in and correct me if I'm wrong. But I would say, yes acclimate them and put them all in at once. That way no one has a chance to establish the territory as her own, and any aggression is spread among all six fish. Monitor them closely for a while; you will expect to see some nipping as they establish the pecking order, but watch out for any extremely aggressive behavior.
Good luck! It sounds like you have a great setup; I'm sure they'll love all the swimming room. What other (if any) fish are in the 55 gallon?