Get the provent a mite and clean and treat the enclosures. Put all rats in a large tub or tubs, with fresh bedding while you do this. Id move them to a different area. Flea bomb the garage or get a big can of flea powder (at any big pet store) and coat the entire floor with it. Leave it there at least 24 hours if you do this instead of a flea bomb. Then sweep or shop-vac it all up but id definately put flea powder along the edges of the walls and in corners and leave it there permanantly to help prevent reinfestation.
As for the rats. Dawn dish soap is what vets recommend bathing puppies and kittens in when they are too small or too young for flea treatment. This will work for your rats too. Provided they are not mean and try to bite you as you bathe each one. Id get 2 buckets. One with warm soapy water (dawn) one with clean warm water to rinse them. Have a friend help and make an assembly line. Wash a rat (rub entire body down well), rinse it, put it in clean tub of bedding, etc etc..
Then put them back into their regular enclosures that have been cleaned, disinfected and P.A.M'ed.
i wouldnt use a flea bomb or expensive flea powder i use sevin5. dont use it on your snakes or let them breathe it