i think one sub is good enough for me but it depends on what your aiming for. sq or i'm that loud annoying guy and my trunk rattles. i'm sq person so i would set it up like this.

so here's my two cents

stick with type x components in front and no rear fill for a such a small car. if you are inclined to have back speakers i would just put some cheaper type s coaxials, plus you save like 400 bucks. i think it would be very very bright in your car with another set of tweeters that close to you.

i would go with a 10in sub or even an 8 so that you don't have a choppy sound stage, but most people won't notice it. also i would have a custom slotted box for it.

my roommate just go the new alpine pdx 4channel and i'm very suprised by how nice they are and how small it was and i do like how they're stackable.