Once again, I'm late to the party, and the guys have already given the best advice! :-[ I'll throw my 2¢ in, anyways, since I'm here.
Steer clear of the little butts! If they are under 7 lbs, they probably aren't bone-in Boston butts anyways. That is the ONLY pork shoulder cut I will use for pulled pork, and it's universally agreed to be the best cut for pulled pork. Stick with 7-10 lbs; you'll get the absolute best results, and will love it!
As Gregg mentioned, brining pork butts adds a whole new dimension to them! I will never smoke one, again, without it (it makes THAT much of a difference).
Temp? I have played with this a LOT! What I've found, is around 190-192 internal temp is ideal. Once the butt starts climbing in temp again, as it exits "the stall" (which can last for several hours, usually in the 170s or 180s), the "magic" tenderizing is done! As you get up around that 190 range, the meat will be firm, but very tender and moist! I find taking butts above 195-200 makes the meat more mushy, which isn't good, imo. I like my pulled pork to have a little meat texture, but be tender. At 190, the bone will slide out clean, which is really what you're looking for.
Rub: Use something that has some sugar in it, for the best bark! It creates the Maillard Reaction, in which the sugars caramelize and meld with the meat. Hard to get, without sugar. For off the shelf rubs, I love Famous Dave's Rib Rub and Bad Byron's Butt Rub. Both are great on butts!