As I interpret 14 CFR 61.63 (Additional aircraft ratings ...), particularly this part, an applicant for a category add-on rating:

(4) Need not take an additional knowledge test, provided the applicant holds an airplane, rotorcraft, powered-lift, weight-shift-control aircraft, powered parachute, or airship rating at that pilot certificate level.

it appears that a category add-on applicant is required to take the Knowledge Test if the applicant does not already have one of the specified certificates AT THE DESIRED CERTIFICATE LEVEL. Sorry about the caps.

If this interpretation is correct, if an applicant for a commercial add-on category rating has only a private pilot certificate the applicant is required to take the commercial Knowledge Test appropriate to the rating desired.

Is this interpretation correct?