I'm renovating my basement and have a pretty fundamental question about the
romex cable I'll be running throughout the space for lights and wall jacks.
I'm having my old Federal Pacific breaker panel (150 amps) replaced with a
new Cutler Hammer 200 amp box. Have hired an electrician to do that work for
me. But I wondered why I'd bother with 15 amp circuits (14-2 romex). Is
there a reason not to simply make all three of the circuits I'll be creating
for the new basement 20 amp circuits, using 12-2 romex? Or is there a
fire/shock hazard caused by using the higher amperage circuit breakers on
those standard circuits.

Also, does on GFI protect the entire circuit on which it is located or do I
need to install more? My basement, like so many, sometimes gets wet when the
outside drains are blocked.