Nokia N900 is not a phone, and its missing hildon-desktop which means it does not have a GUI to work with.

In other words you're pretty much screwed unless you know how to work with mounting and chrooting into maemo from nitdroid.

Also, flashing using the combined firmware will lose multiboot and nitdroid. #maemo on IRC recommends that you flash both eMMC and combined instead of just combined. You are forewarned.