You'd think that with the amount of time spent during ME training or checkride with one engine inop, the SE part would just be automatic!
61.129 lists the aeronautical experience requirements, and specifically delineates where "an airplane" or "single engine airplane" is required. Like 5 of the 10 hours of complex time could be either, but the x-cntry and solo parts need to be SE.
If you already have a Commercial with AMEL, then you're looking at 61.63(c) for Additional class rating, which basically says you don't need to repeat training or knowledge test just for an add-on like ASEL. The Practical Test Standards has a matrix table (somewhere in the front of the booklet) that shows what tasks would be required on the checkride for an add-on.
If I understand your situation correctly, it would probably be best to take the checkride in a multi-engine, then as a holder of a Commercial AMEL, the addition of SE will be relatively easy, and would require the least amount of additional aircraft time and training. (That would certainly be the way I'd do it.)
Hope this helps, good luck.
Edit: OOps!, zondaracer beat me to the post while I was typing!