Sounds a bit like War Against the Chtorr (not in a bad way and the Chtorr books might be useful for ideas)
https://en.wikipedia.org/wiki/The_Wa...nst_the_Chtorr
Some other resources that might be useful for you: -
This page is for the videogame series but the download section has an offline version of the first game's UFOPedia. I thought it might be useful
http://www.xcomufo.com/x1dl.html
This PDF was for converting the first X-Com game to the Savage Worlds rules
https://web.archive.org/web/20111012...avageX-Com.pdf
This download is the quickstart rules for a game that was apparently not successful on kickstarter but supposed to in the vein of X-Com (I haven't checked it myself yet)
https://www.scribd.com/document/2508...act-QuickStart
I do have a question about the precursors, you have a paragraph giving some details but it seems unfinished. Specifically, it mentions that they have proportionally longer limbs suggesting... there's no more info, from there it goes on to mention enlarged canines.