The way I see it, a system is a classic if the company that owns the console no longer produces it, and the system still warrants a purchase, and is still worthy of play. So I can consider my TG16, Jag, 3DO, etc. classics; whereas I don't consider my, oh... 32x (I know its an addon, but it's late, and I can't think) to be a classic. I instead consider my 32x to be a collectible, which sits and collects dust, but I won't sell it.
It's almost to the point where I can consider my Dreamcast a classic. Sega not producing consoles... sad, sad times we live in.
Anyway, I digress...
As for the SMS and NES, I agree that they go in the same generation as the 7800. Afterall, same time period, even if they were technologically superior.