You need to understand the difference between 'licence' and 'rating'. In JAA land, the CPL is a licence, to which ratings are attached. It makes absolutely no odds on whether you train for a CPL on either a SEP or MEP......the licence is what you will gain. At some point however, assuming you want to fly multi engine aircraft commercially, then your going to need to do the MEIR, which will also require you to do/or have already gained the MEP rating. So....you can either do the ME rating as part of the CPL.....or part of the MEIR course itself.....or you could just do it as a stand alone rating.