This is the time attack mode where you can play as any character. you can still make allies in this mode, and other than never changing your own moveset, it also adds a bunch of Smash Run style powerups you can collect in the stages.
Not gonna do this right now, just wanted to see what it was. I'll try out all the other modes while high though.
The Ultimate Choice
This is the Arena of this game. Level 5 seems to be the normal story mode difficulty. It wanted to start me on 1. I almost beat this on my first attempt. Died from Hyness rolling around in a ball. I was hoping Star Allies Go would also have a difficulty setting but I guess not.
Star Slam Heroes
Basically a tiny rhythm game. The higher difficulties let you charge up with the power meter multiple times to get higher distances.
Wow this one is really good... it's like a Mario Party minigame with 3 different speed levels, and 3 highest level has gordo enemies that move between the 4 players.
And yeah, I've confirmed that these extra modes count towards file completion. I'm not sure if the Guest Star mode only needs to be beaten once with any character to count towards 100% or if you need a ranking for every character... With Ultimate Choice I'm guessing you just need to beat each difficulty level once with any character.