Some fit depending on the size. They might need custom mounts, some will fit if you mod the plastic mount to fit the larger speaker. 6" fits perfectly without modifications.
Just to add to that... many 6.5” speakers are imposters!!!!
Well, sort of. They are listed as being a 6.5” but are in fact a 6” but have mounting tabs for a 6.5”.
Easiest way to tell is to look at the speakers mounting holes. A TRUE 6.5” will only have 4 screw holes whereas a 6” will have a lot more for different mounting options.
I don’t recall any true 6.5” being a drop in replacement. They’ve all needed the white plastic adaptor to be modified to some extent. Sometimes, I’ve just needed to cut off the water bucket and others have required me to run the jigsaw around the mounting plate to make it fit.