I guess that Shivans might be based off the same classic SF (pretty current at the time of FS1). While pretty well written, Shivans are quite generic in their original, FS1 version when you get down to it. FS2 Shivans are much more mysterious, but also somewhat generic as far as cosmic horrors go. We could probably trace both Reapers and Shivans back to some Lovecraft's story (note, I haven't played ME2 or 3, so I only really know them from ME1 and occasional spoilers). They have plenty in common, but it's just that they use the same cliches (the Reapers are consciously written to reference classic SF, while Shivans most likely weren't so cliche when they first appeared).