I wouldn't hesitate to add it to Shadow World. The Magic Isle and eveb some of the more obscure continents will have magical practitioners that Emer & Jaiman have never seen. An elementalist would not be the issue, it might be some of the concepts behind them like triads & elemental beings. But, in my mind SW has always been a "magic as technology" environment, so concepts like those presented in the Elemental companion seem very compatible, and we all know that there are a multitude of powerful Demons/Angels/deities and other being that have influence over the people of Kulthea, so i wouldn't worry too much about that.....I always suspected that the Earthwardens were some sort of Uber Druid/Elementalist....maybe that could be a hook for you
