First of all, any question regarding if it's possible hardware-wise is null. The
official site confirms you can use up to 8 GameCube Controllers and many other control options.
So at this point it's all up to the players if it's a possibility. Personally, I think it is. Competitively? Hard to say, it's a thing you need to experiment with. It's completely a new thing to Smash.
There are also several other variations you can try out:
- 2v2v2
- 2v2v2v2
- 3v3
- 4v4
Depends if friendly fire is enabled for this. It doesn't work when it's off.