Well, I think at the moment there are not that many stages that are enjoyable for 6 players. Most of them are too small to have a fun match with 6 players.
Additionaly there is no possibility to turn off stage hazards.
I think some hyrule sized stages should be added in the future.
Or maybe a stage editor? But I think that these are quite difficult to program.
I'll add some 6-player stage ideas under this post in the future