In the photo below, are those the little stubs that need to be sheared off for the center cap to fit? Do I need to do the whole set or the rears only?

That's them, be careful not to take too much off as it will cause the centercap to remain loose and rattle. I did all 4 of mine

Just pull them out. Worked for me.

Pull the tabs off before you cut it down flush, then reinstall the tabs back in. I cut it flush so you can have more threads to tightened down.