Does Multi-engine Commercial include Single Engine Commercial

I am a SEL private pilot with Multi-engine and Instrument.  I did not take my instrument checkride in a multi-engine plane so I have always assumed that my instrument rating was only valid for single engine only.  I would now like to add my commercial rating and while I am at it tie together my multi-engine and instrument ratings, so my thought was to do the commercial checkride in a multi-engine aircraft.  However, the instructor I spoke with said that if I did that, then the commercial rating would not be valid in single engine aircraft and I would need a separate checkride for single-enginge. 

I guess military pilots have this problem when they already posses commercial multi-engine ratings but have little SEL experience.  But does it apply to those of us who have done most of our training in SEL aircraft?  I guess the problem my be with chandelles and lazy 8s etc.   Could I just go out with an instructor in a SEL aircraft to log proficiency in these maneuvers, or would I need to do these maneuvers in the twin during my checkride, or would I need to use 2 aircraft during my checkride, or would I need to take 2 checkrides?

Thanks,

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