Considering all the races work together (more or less) to battle the Xenon, an alliance of the X races would even be justified in the backstory... as long as the Xenon and Khaak aren't part of it obviously.
And while it wasn't part of the official campaigns, a revival of the old anti-Xenon alliance to battle the Khaak would also make a lot of sense, since the Khaak attack all of them in equal measure.
"Argon (Long Lost cousins of the Terran's)" Weren't the Argon a mix of a local species and the survivers of the Terran fleet?
"Paranids (Race that thinks every other race is unholy and unclean)". Not all other races, only those who's number of eyes isn't dividable by three. They themselfs have three eyes of course.
As for getting X3, or any game of the X series, make sure you know what you are getting before deciding. The official Motto is "Trade, Fight, Build, Think", though in matters of what you do in the game it's more like "Trade, Think, Build, Fight". Terran Conflict did enable to player to succeed with far less trading than the predecessors, but there trading was absolutely necessary (especially in X-Beyond the Frontier, were you started unarmed with 100 credits).
If you like trade simulators and building your own trade company, go ahead. In many ways it's like a sandbox MMO without multiplayer.
If you like FreeSpace, WingCommander or X-Wing like space sims or expect a deep and involved story campaign, don't bother.
