They're certainly "alive" but there's some major difference between outsiders and other living things. I don't remember where I saw this(or maybe it's only in my head), but I think I remember reading that whereas humans and other non-outsiders have a dualistic self, body and soul, whereas outsiders have only one unit that comprises both body and soul.

That said, I wouldn't see any problem with allowing demon-skeletons or something. Actually, for some of the more human-like outsiders, I might allow other undead templates. Vampiric Succubi would be an interesting opponent...