So I was thinking.
People are saying from the video of the seiyuus playing that you start with 3 songs. Maybe it's just that quintets can only do those songs, since they didn't make motion data for the other ones?
Doesn't make sense to me. You can only have guests appear if you do one of four songs? I'd be pretty let down.
Sometimes people get pre-release trial thingies to do pre-release stuff, so any inconsistencies might be able to be explained with that. In fact, I think the way game reviews go, they get a pre-release version of the game with a list of stuff that they say will be changed/fixed on release. This explains why a review can be out so close to release, but also places restrictions on reviewers: sometimes they can't comment on what the company says will change.
... Or so I remember reading once.