The idea behind exclusive characters have always sickened me, and it would be terrible in a game like Smash Bros. A slightly smaller roster on the 3DS I wouldn't mind, but if the 3DS got characters that the Wii U version didn't get then I would be pretty furious. And about Ice Climbers, they're probably gonna figure out how to integrate them somehow.
Olimar shouldn't be a problem, I'm pretty sure the problem isn't about having too many things on the screen at once, they wouldn't develop a game like Smash Bros which has countless items and characters which makes tons of objects appear on the screen at once if that was a limitation. Plus, Nana is only one character model, no way that's the problem. The problem is most likely getting the Ice Climbers to both react to a single button click and have them synchronize properly too, or something to that extent. Given that the Pikmins are essentially items with an AI, or at least they can be made to be so, it's not gonna be a problem for Olimar.