Scouts are generally good against everyone, except pyro.
They're also good to break the medic+heavy combo as well.
Now, the trick with the scout is:
You don't want people facing you. Ever. If you're far away, your dodging only means a slight movement. Scouts should jump onto people and constantly make them do 180 turns to try to face you, jump over their heads, shoot, do it again. You also shouldn't be going into groups of more than 2 people unless you have backup. Furthermore, when you pick an area to attack someone, make sure there aren't other people watching your assault, because then they can spam-kill you.
As for the weapons:
Original Scattergun - This weapon is specially good when approaching targets. It has a nice tight arc of fire, so if they suddenly face you, you can shoot and dodge. Plus like the shottie, you can reload shot by shot.
FaN & that new shootie - This is the ambush weapon. Run up to someone's back or side and shoot them once and then while they're in the war, do the second shot. It's insta kill for most light classes, doing around 200hp of damage with both shots. Worthless when far away, so hold tight to that pistol for a missed second shot or people trying to get to you.
Shortstop - This is the mid range weapon. Has the tightest arc and shoots less pellets with each bullet. With this thing you can pretty much ditch the pistol and use some of the fun drinks and the mad milk. Note that it does slightly less damage than the scattergun, but shoots faster and reloads 4 bullets at once, so take that in consideration. This is great for popping hurt heavies out while behind some cover, being able to do around 160hp of damage with the four shots at mid range.
Ps. I'm only a good scout in 5 point CPs for some reason, so if what I said doesn't work in PLs, you know why.