Exactly, there's no such thing as a single or multi engine CPL. In the end you will have a CPL + MEP class rating + IR/ME.
So when it comes to the CPL lessons take as many hours in a single as possible in order to save money ( unless you want to combine it with a MEP rating ). However, 5 hours has to be in a 'complex single'. If your school doesn't have a complex single engine aircraft ( retractable gear and variable pitch prop) then they will probably put you in a multi engine aircraft.
I believe that the skill test has to be in a 'complex single' as well, but someone correct me if I'm wrong.