Actually, the old tyrants look closer to the Xenomorph/Ripley hybrid in the later film (ressurection?).
And you are correct in pointing out that SC came AFTER 40k. Blizzard and GW did work together on SC, but as the rumour goes, they fell out, and Blizzard didn't bother changing much (terran marines), just the names. (no idea if this is actually true or not).
Overall, there is alot of "sharing" in fantasy/Si-Fi, and 40k is no exception.
Back on topic, yes 40k does borw from real-world stuff, you name a piece of fiction that DOESN'T.