I would not suggest having a supply duct in your garage......if that register was open and the air handler was not running it could cause Carbon Monoxide to enter your living space from car, golf cart or lawn mower exhaust.
As to the breaching of the fire separation rating with the attic stair door, that would be correct. It would be a breach. In my home the pull down attic stairs in the garage are NOT fire rated, and only have a 1/4" plywood backing. It is 15 years old. The addition of those vents did not compromise it much more then it was already.
They DO make fire rated pull down attic stairs. They cost more, ($400-$500). They have them in both steel or fire retardant plywood. Of course, cutting vents into these would nullify the rating.
For the record, I am a Professional Firefighter, (IAFF Local 896) and a licensed Home Inspector.
Frank
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The Plantation in Leesburg, just south of you good people. Love being a Floridian!
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