People are winning tournaments with Rosalina and Luma because Rosalina is actually pretty darned good all on her own due to all the disjointed hitboxes on things like her Fair, Side tilt, up tilt, and Uair and Dair. Sure, she's a little floaty and light for how amazonian and tall she is, but she's pretty great, and get insane when she has a Luma. It's not like with the ice climbers where you're out of control of your second climber if you get divided or it gimps your jump when you don't have a second one to belay with. I've been training with Rosalina and Luma, and even without a luma, I still get the best of my friends offensively.
Olimar is actually the only character with a major difference between the Wii U and 3DS version... see, in the 3DS version, the Pikmin are locked at 30 FPS movement rather than 60, and that made all of his attacks garbage-y and slow. on the Wii U, it's MUCH better, but not enough to really make him a stronger member of the cast.
Shulk actually got a buff int eh 1.4 patch where his moves have less lag or have more bite to 'em...!