When I got my wild rat babies, they both had fleas. When they were old enough, all I used was a flea shampoo for kittens (I think, I did ask the vets) and flea-powdered the carpets. I never had a problem after that.
EDIT: I would say though that was only treatment on two of my rats. I kept them well QTd in another room of the house so none of the other rats (or cat, or dog!) caught from them. I wouldn't know whether the same treatment would work as effectively if you have quitea few with the same problem