It is common for an author to experiment with different genres, art, and media. But at the same time, it's also common that the same author uses different names for different demographics.
You should distinguish between the writer and the pen name he/she uses, and between individual authors and groups. CLAMP is a collective and they're changing its components periodically. On the other hand, Masashi Kishimoto is an individual author (that it doesn't mean that he hasn't any assistant).
Pseudonyms are common in this industry where we can have an individual author Sumomo Yumeka writing yaoi with this name, seinen manga using the pseudonym Mizu Sahara, and writing shoujo manga as Sahara Keita. Any pen name is clearly associated with a single demographic in this case, but the author herself actually write for different demographics. It is the same with Ken Akamatsu (shounen) aka Awa Mizuno (hentai doujinshi), so it seems that where publishing constraints are very different, authors can use a pen name to differentiate their works.
Collectives and circles like CLAMP seems to explore different genres/demographics more freely with the same name, probably because the authority is shared among the members and the collective nature of the name is clear to readers.