Shivans aren't terrestrial. They don't invade planets, they bombard them or ignore them completely.
Yeah, although the game makes it pretty clear that Shivans don't like landing on planets, the bodies on the planet surface are apparently dead pilots/crewmen, although they aren't distinctly recognizable as such. Not that an organism would remain recognizable at all after descending from space, but this is FreeSpace.
Look at
Hades' pic. We have some weird metal cowling-looking contraption to the right. It looks as much like a Shivan leg as anything.
If that was shivan, you would see more from it.
Random fluff text by Dr. Hargrove from the first Knossos command brief: "Although Shivans exhibit considerable diversity as a species..."
probably wasn't paying attention when they said that, but it's a canon statement.
