Can all fleas potentially carry the plague bacteria?

No. Only fleas which have been exposed to rats or other animals infected with the plague bacteria (Yersinia pestis) can transmit it to humans. Y. pestis colonizes the flea's gut, replicates, and blocks the flea's gut. The fleas then feed voraciously, regurgitating the bacteria into the bite. Only a few organisms are required to pass on the infection. Most cases have been in in CA, AZ, NM, & CO.
Probably yes. Rats, prairie dogs, chipmunks, squirrels, deer mice, etc..., and the fleas of all these animals can carry the plague. Other animals, such as rabbits, also can have the plague. After pet cats and dogs catch the plague, their fleas can become infected and transmit the bacteria also.