how are you supposed to get to the back of the rivets to cut them.. you can't just reach back there and even if you could you can't even see what your doing. Drilling them out doesn't take much. I've changed the whole stereo in my car.. new speakers, front and rear, subs, deck, amps.... wiring.
As for fitting the speakers just some nice 6 1/2" (its the same for the front, and those too are rivted in and have the same sort of mounting mechanism as the rear). Go to any stereo shop and ask them for some ABS plastic. You can trace the outline of your stock speaker and cut it out so that it will fit the same, and then cut a whole just big enough for your new speaker to fit in.. but don't make it too big cause you gotta mount that speaker to the plastic and then mount the plastic to the car.
For the rear my speaker mount is somewhat like the toyota one... when i went to the stereo shop i brought my stock speaker and they gave me two sheets of ABS plastic and two rings the size of the speaker so that it would hold the speaker out like the stock one does.. if that makes sense. It fit perfect.
Its really not hard to do all that stuff, you just need the right tools.