Personally I love the fact that you can use a single Joy-con and that's how I will play.
Eww, good luck.
I played Smash 4 on the Pro controller, and I'll play Smash Ultimate on the Pro controller. Over the course of Smash 4's lifetime I've seen many of my friends move from Pro to GameCube controller, but despite the trend growing increasingly prominent I never quite understood it, I find the Pro controller to be quite objectively superior, its only downside being potential lag at bigger events. Of course, with the Switch Pro controller sending data through USB, that's no longer an issue, so there are no downsides anymore - and that's not even getting into how good the controller is as a whole, I absolutely love it. I can see it being a little weird to get used to playing Smash on it at first, because of the right analog position and the size of the face buttons (a notable thing for me, because I use everything on face buttons: attack, special, shield and grab), but I'm sure it'll come good in the end. If anything it can't possibly be worse than the overrated GameCube controller.