well to answer all your questions
the difference between attack and special attack, is actually very simple.
thunder punch, is an attack, because it makes physical contact.
thunderbolt, is a special attack, because it doesn't.
you can identify weather it is an attack or special attack by the little icon in the attacks menu.
if it's a sideways red rectangle with a yellow star shape in it, it's ALWAYS a physical attack.
if it's a sideways blue rectangle with 3 light blue circle in it, it's ALWAYS a special attack.
( the gray-ish rectangle is for attacks that are neither, for attacks such as dragon dance, and hypnosis, or other attacks that don't deal direct damage )
as for the Eevee with an Impish nature, it's not really a great nature to have n it, to be honest.
but since it's plus defence and minus special attack i'd suggest either Umbreon or Leafeon, since the both are ( kinda ) bulky and don't use their special attack stat.