I would put a 4 cct sub panel in the garage, then the plugs can stay 20 amp. It just isn't worth the argument. Having said that, it would be code compliant to add the light to the existing circuit. In fact, the light probably presents less of a hazard than the receptacles. To be really safe, you could protect the light with the load side of the GFI.
Here is my usual rant: Electricians need to be able to take the rules at their normal meaning.
Where I am, we once had to track where inspectors worked, because they "interpreted" the rules differently. I even had to explain to one customer that her wiring had to be different than her friend's, because the inspectors were different.
Because the inspectors "interpreted" the rules differently, if one was away, no other inspector wanted to answer questions about work being done in that inspector's area, and there tended to be a lot of questions.
Even when inspectors agree on a nifty application of a rule, it puts everyone in a position where they need to phone an inspector when something new comes up. By the way, some inspectors don't like the number of questions they get, either. He didn't actually say that, though. What he actually said was "go ahead and ask your question, everyone else does."
When the rules suck, CSA should re-word them. That's their job. If my work were as sloppy as some of the rules, the inspectors would reject it, just as they should reject sloppy rules. I'm absolutely against sloppy applications of sloppy rules.
It's pretty cool to be able to phone an inspector with a question. It isn't cool to ask the definition of a dwelling unit, when it's clearly defined in the front of the book.
If it complies with a strict interpretation (or any other kind of interpretation) of the code, and it isn't an unreasonable hazard, it must be must be what CSA intended, or they would amend the definition.
