yes, definitely merge skyline discussion and GAD..
i mean most have a bit of an interest in all types of cars, even if their main passion is skylines. Splitting off into sub categories just decreases the traffic to all, and means you have to click several times to read everything, rather than just one place (as merli pointed out).
As long as the post traffic isn't too massive in one area, i think just having it as one area is more useful - even if it is a bit "general".
Some should stay separate and a good idea, others should be merged I reckon. As to which.. hmm I don't quite see the point of having "classic Skylines" , "other nissans", and "all other rb powered nissans". Shouldn't this be just one "Classic Skylines and older Nissans" or similar?
In my experience having too many sub-forums can just kill discussion off in all of them