Can this steel be put over shingles, or do they have to be taken off?

Yes, the exposed fastener panels can be installed over a layer of shingles. The bare minimum preparation is to make sure the roof is still flat (no corner of shingles turning up, etc.) and to put down a layer of underlayment. The panels will contour to any sags in your decking though and you can "dent" the metal when installing the screws because you may hit uneven pressure points if hitting a waterjacket in the shingle below. The other way is to lay down lathing across the roof every two feet (from gable end to gable end), lay down your underlayment and then screw into the lathing. This will make help with the potential issues identified above with going directly over shingles.