I don't think that 3D Marios are good fit for co-op. 2D is more action and frenzy and 3D is more calm.

And can you chose Luigi from the beginning or is like the 1st part where you have to get the super-mega annoying purple stars >:(

Does not say yet  so I assume no. Would be nice if you could use him from the start  and put Mario in his role, or something like that.

From the recently released vids it seems like you'll encounter Luigi at certain points in the game and you'll get the option to switch to him then, not whenever you want... And Miyamoto said in an interview a while ago there'll be 240 stars in the game, which I take to mean 120 for Mario & 120 for Luigi on a 2nd playthrough, like in the original Galaxy. If that's true, then being able to switch between Mario & Luigi at will would be sort of pointless.

Also, IGN put a preview up, as they recently received a copy of the final game, and the guy that wrote it played the game for several hours with no sign of Luigi...