Endoparasites would be difficult to implement, and scarab-size ectoparasites would be somewhat easy to pick off, I think... Then again, I would not know the actual size of a scarab, as they don\'t exist, except in Age of Mythology, in which case they\'re the size of two houses... Really, the discussion in the Parasites thread pretty much covers why most people dislike the idea of parasites.
As for the story... Where does it take place? The surface of the PS world, or Yliakum? Or somewhere in the Stone Labryniths, or somewhere else entirely? If on the surface, symbionts wouldn\'t be that much help. It will be a looooong time before anyone met a symbiont. If in Yliakum why doesn\'t some other group notice the portals? If in the Labryniths, it would make perfect sense, other than the slight factor of the war... Where in history does it take place, and who is involved?
According to the Setting page, the races other than the Kran and Lemur were transported from other worlds. Perhaps the symbionts could be creatures that somehow \'hitched a ride\' or sneaked through, and entered the PS world. Now comes the combining of life forces, except some do it with sentient beings, probably in secret, and others with creatures of the Stone Labryniths.