If anything, Stargate may have stolen from KA, but has anyone watched Star Trek: Wrath of Khan?!
Or the ep with the things in TNG?
To be honest, I think the idea of parasites is just one of those things that's been passed down through the ages in Sci-fi. I've ran into it so many times. There were things with gills out of the backs of people's necks (TNG), the Trill (TNG and DS9), things in people's shoulders (some book series my dad read), the latest non-Dresden Butcher novels (kind of obvious there), Venom (sp? Spiderman 3), some manga or something called Parasite (duh), and I'm sure plenty of other examples of parasitic aliens taking control of people. Even parasitic aliens taking control of people with the best intentions or working to fight against the stance of the other aliens, or whatever.
(To be honest, I'm an Atlantis fan. I haven't really watched SG-1 and am sort of playing catch-up with Atlantis, so I have a very, very basic understanding of the Gou'ld and the Tok'ra, which comes from my friend...
I really need to steal my other friend's Atlantis DVDs...)