John,

You're right it doesn't seem to be exactly clear about when you can fly solo when adding a new category and class to an existing certificate. But being an examiner, I run into this frequently. The short answer is that you must satisfy the instructor that writes the endorsement for PIC privileges in the new aircraft.

The endorsement will be for PIC privileges in a category and class aircraft for which you do not hold a current certificate. It will have limitations regarding carrying passengers and other limitations the endorsing instructor thinks is necessary.

Once you have a pilot certificate, Private or Commercial, you will NEVER be a student pilot again and so do not have to complete the student requirements listed in 61.87 for the various aircraft category and classes. However most instructors will evaluate your knowledge/skill against those requirements prior to issuing an endorsement. No knowledge test is required until you want to actually take the practical test. Then both the computer knowledge test and the practical test must be passed to add the new privileges to the certificate.

Prior to taking the add-on practical test you will need to receive and log the areas listed in the appropriate section of 61.121, or at least show proficiency in those areas.

FAR 6.63 discusses the minimums for getting an Add-on rating.

The commercial certificate MAY be issued without an instrument rating, but it will again have specific limitations.

I hope that helps,

Mike B.