Okay, to answer your questions about Luigi being a playable character here is a piece of information from this month's Nintendo Power Magazine:
It's surprising how Mario's scaredy-cat little brother, Luigi, keeps finding himself in the spookiest places. Ghostly Galaxy is packed with giant Boos, trap doors, and one green-suspendered, knee-trembling sibling. But saving his bacon is worth the effort-once you get Luigi safely back to the Comet Observatory, he'll help you out in your quest to find more-tricky stars.