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Why aren't you going through IAH with CO to CZM? How did you get a routing via YYZ? Going north to go south seems like a waste of travel time.
For international passengers in transit I'm not sure you do have to clear Canadian customs. To be sure, I'd contact CO directly and ask. Possibly if your flights arrive and depart in different terminals you would have to clear, if not, I don't think so. You'd just follow the signs for international passengers in transit. If I remember correctly, you'd have to be arriving and departing in Terminal 1 for this to apply.
Yes, it makes no sense but that is what I get with my frequent flyer miles.