You can have a Class 1 medical with a PPL but not a Class 2 medical with a CPL.
I would wholeheartedly agree that anyone considering professional flight training should get a Class 1 medical first, the earlier the better (as the renewal standards are less restrictive than the initial standards and it's easier to get the Clas1 before the body starts falling to bits).
But of course, the probability is that the Class 1 medical privileges would expire well before the student completed training and obtained the CPL; falling back to Class 2 privileges would allow the student to complete training with a Class 2 medical, just getting a renewal to the Class 1 for the licence application.
I think though that the spirit of the law is such that a CPL holder with a Class 1 medical which has expired (for Class 1 privileges) could exercise PPL privileges provided that the Class 1 medical was still within the Class 2 expiry date.
