Can porcelain veneers and crowns be shortened once they are already cemented in place in my mouth?

Absolutely they can be shortened! In most cases it is only by microns not inches so the minimal amount should not affect or compromise the strength, margins or aesthetics of the crowns or veneers. If the dentist feels he has removed a significant amount he can send crowns back to be reglazed, veneers would have to be shortened after they are cemented and a rubber wheel can buff them up very nicely as well.

Can porcelain veneers and crowns be shortened once they are already cemented in place in my mouth?
They can be shortened by grinding them down but it is not recommended as they will just not look as nice/shiny. The dentist should have given you a mirror to look at them before he cemented them in place.