Incidentally, Crutchfield'* "fit guide" is notoriously bad for our Bonnevilles. Although Crutchfield is generally great for other car audio things, especially installation kits and instructions. But I think you can do way better than to just trust whatever a Crutchfield rep on the phone tells you will fit or not.
My Bonneville SE came from the factory with no rear speakers, so I didn't have the plastic mounting rings that the factory 6x9 speakers attach to. Crutchfield phone guy told me I needed these:
Short version: No, those won't go in our cars. You can see the one that does if you scroll that page down a bit, and also on page 2 of this thread that chronicled my install:
My 6x9 Boston Acoustics speakers did not fit into the stock plastic mounting ring, because the metal speaker basket was slightly wider than the hole in our plastic mounting rings. This might be along the lines of what your Crutchfield rep was saying. Although ten minutes with a Dremel tool widened the opening so my BA'* dropped right in.
I wouldn't put round speakers in the back where you have oval 6x9 openings, when you can just get 6x9 speakers in the first place. Doesn't seem to be any critical need to put a smaller round speaker in an oval hole.
97 SE, teflon wiper blades, Dunlop Sport A-2 tires, Leather steering wheel cover, Pioneer DEH-P480MP mp3 CDRW head unit, Pioneer 12-disc changer, iPod hookup, Boston Acoustic 3 way 6x9'*, Boston acoustic separates in door pods, Alpine 4 channel amp in trunk. Magnaflow dual outlet muffler and Ractive stainless tips. Autozone rice pipe and K&N 4750 intake kit. Clear corner lights. Energy Suspension end links.