If you have fewer than four cards in your library as Clone Shell's first ability resolves, you'll look at all of them.

As Clone Shell's first ability resolves, you must exile one of the cards you look at, even if none of them is a creature card.

As Clone Shell's second ability resolves, if the exiled card is not a creature card, it simply remains in exile face up.

If you gain control of another player's Clone Shell, you won't be able to look at the face-down exiled card. However, if Clone Shell is put into a graveyard, you will turn that card face up as the ability resolves and, if it's a creature card, you will put it onto the battlefield under your control (even if you don't want to).