How do ratings work with pilot certificates?
This answer applies to adding a new aircraft category onto your existing Commercial Pilot Certificate in the United States.  Consult the standards of your country's governing body for aeronautics if attempting to do this outside of the USA.

To address the second paragraph, once you hold a Commercial Pilot Certificate for the same category and class of aircraft as your private certificate, you are considered to hold a Commercial Pilot Certificate only.

In regards to the third paragraph, yes additional training will be required to fulfill the requirement of 61.63 (b) for the addition of an aircraft category rating to your CPL.  You will be required to meet the aeronautical experience requirements listed in 61.129 (c) and receive an endorsement in your logbook to take the practical exam in order to add a helicopter category to your CPL.  You will not be required to take the commercial pilot knowledge test again per 61.63 (b).