The only real requirement is that there should be an Easy or Normal diff in the mapset (for beginners). And that the difficulty spread isn't to large.
So spreads like Easy/Normal or Normal/Hard is rank-able. But, having a Easy/Insane or Normal/Insane isn't (since
But its preferred but not required that you have a Easy, Normal & Hard with insane being optional at-least. Of course, some of the slower songs don't warrant a hard or its really hard, to map a "Hard"
Having taiko diffs or any of the other game-modes in a mapset is optional and not required. But if you say, map a "easy" taiko, you will have to supply a harder version too.
I hope this clarified it for you somewhat.
(Granted, I've been away from the mapping scene for awhile, but things shouldn't have changed to drastically.)