You can do that no problems on the CPL but there are a few considerations. Firstly, I think Cabair do their CPL skills testing on the multi, so that would give you an MEP class rating on the licence, and to go back to flying a PA28, you will need an SEP class rating which you will have to do a few hours training and a mini flight test for. Once you have this in place, you need to condider currency on type before going up. Some flying clubs might require a certain number of hours in the preceding 90 days for you to be able to go up solo. So, bottom line is there are no problems hiring out light aircraft on the CPL as long as you are suitably qualified and current.