1) An "SP" deck is a deck that is based around SP pokemon, like Legos or Luxape or something like that. It has the main SP line (the base and the lv x a lot of time) and SP trainers such as Poketurn, energy gain, power spray, and SP radar, along with Cyrus's conspiracy for fetching the inventions (i know not all SP decks have those trainers, but a lot do it seems). If a deck contains SP pokemon, then i guess it could an SP deck.
2) When you have the special dark attached to a dark pokeman, it will do an additional 10 damage to the defending pokemon only if the attack originally did damage to begin with. If an attack does no damage to begin with, then a special dark won't give it an extra 10. And special darks can stack, if you multiple special darks then each of them will give an extra 10 damage to the attack that you choose, if it originally does damage in the first place.
Hope i answered your questions well enough.
EDIT: Ninja'd yet again.