The CPL is a licence. Multi Engine Piston, Single Engine Piston, Instrument Rating are ratings that are added to a licence.
If you hold a CPL and then do your MEP rating, the MEP rating is added to your CPL. The actual MEP course is the same regardless of the level of licence held.
And billie bob to clarify your comment. You need to have completed and passed your MEP rating prior to sitting the IR test. However you only need to have completed the training for a MEP rating prior to starting the IR course.