I guess I was just wondering how they came up with the different idol combination for certain songs. I'm assuming they didn't do any new recordings for this game.
Like the first song in the video was Mahou o Kakete. It seems all the idols were singing it, but there wasn't a prior recording of all of them singing together whether it's the master version or the short version (at least according to the wiki).
So, I'm guessing they took the game version (short version) of each idol and combine it into one song, so that they're all simultaneously singing together. (It sounded like they were all singing every line together, but I could be wrong).
As for Little Match Girl, they could have done something similar for Chihaya, Yukiho, and Makoto (since there's no prior recording of all of them singing together that song). So, I was thinking they take the game version of each idol and combine it together, so pretty much the Stage for You version for that song with those 3 idols.
Anyways, I know I'm thinking too much into this. So, very sorry for the confusion and maybe stating the obvious (but could be wrong about everything I just said). I'm sure it's going to be a really fun game.