I'm operating under the assumption this is OU, since you mentioned picking OU stuff. If it's supposed to be for VCG, which means doubles, I apologize as the team composition's going to be completely different.
For Scizor, I would recommend a bulky Defog set as between Charizard and Thundurus you're really not a fan of Stealth Rock staying up. Something along the lines of maxed-out HP and SpDef with Defog and Roost, and then probably U-turn and Iron Head/Bullet Punch.
Pair that with a physically defensive Amoonguss: Black Sludge for the item, and then Spore/Giga Drain/two of Sludge Bomb, Toxic, Clear Smog, Foul Play and Hidden Power Fire.
As far as Keldeo vs. Swampert is concerned, Keldeo provides a very strong offensive option that Swampert can't hope to match, but it's not exactly the bulkiest thing in the world. It really depends on whether you want a stronger team on offense or defense, but if you opt for Swampert I'd look for something that might be able to use Wish and Heal Bell in support instead of the Amoonguss pivot. I'm not sure what you mean by Barbaracle or Walrein or Scrafty, but all of those are completely outmatched at OU level and you shouldn't worry about seeing them or using them.