1. I'd say 4g and under are Pico and 5g-30g (Some say 35g) being a Nano.
2. Volume of display is what's advertised. Most hobbyist will include Sump/Fuge volume separately. Most will say they have a 200g reef w/ 50g sump, not 250g reef (as an example).
3. Yes, technically. (I'd love to see that!)