Someone with the base system has to help you, with regard to the exact size of the rear speakers. I have a 10-speaker system, so I don't know what the base system sizes are.
But with regard to the speaker selection, the impedance of the replacement speakers should match the impedance of the factory speaker it is replacing. The impedance of each speaker is shown on the wiring diagrams in the factory service manual, and typically is printed on the back of the speaker.
Free factory service manual download, courtesy of Gary Doug:
The base 4-speaker system uses 6.75" speakers (most 6.5" speakers will fit) in each of the locations in your vehicle. I'm not certain on the impedance of the speakers, but from my recollection the base level system speakers had higher impedances (8-10 ohms?) that those of the Monsoon system. The head units are interchangeable between base level and Monsoon systems. (Monsoon-equipped systems had appropriate markings on the radio faceplate, but there are no internal differences between the two systems.) That being said, you would be able to replace the factory speakers with 4-ohm units, the most common impedance among the aftermarket choices.