Yes, your understanding is correct but, given the CPL course is only a few weeks full-time and you wouldn't want to leave too much time between training and test, it's a risky strategy.
If you're doing ATPL exams then all of them should be passed - the CAA will know for which ones you've been registered. There's no such thing as CPL only subjects; the CPL syllabus is just a cut down version of ATPL (e.g. Navigation comprises Gen Nav and Radio Nav).
Cheers
Whirls