I add the FiveCore material to the army books, mostly as an extra benefit for Clash and Laserstorm players that enjoy FiveCore as well.
I don't think FiveCore is a game that really benefits from "army books" as such, but I do want to have some neat material in there for everybody. If I ever do a scifi RPG, it'll get character notes in the army books as well.
I just finished up the Precursor FiveCore section with a random colony generator, and 5 character-types that can be dropped into any scenario.
If you want to just add a space elf, they get the following special rules:
When Dashing, if you roll a 1 for move distance, roll again and use the new roll.
If the new roll is also a 1, decide to either not dash or to move the extra 1".
If a Brawl ends in a draw, the precursor immediately Bails, as if a 6 had been rolled on a Shock die.
This overrides all over "on a draw" type of abilities.