Honestly, that makes Predators sound either pretty goddamn stupid, horribly overconfident, or a bad case of fan wank in my book. You don't take a pellet gun to destroy a tank, and you don't take "moose-hunting equipment" to destroy nests of freaking Xenomorphs. Considering the terrible success rate on those missions we've seen of that and what just one Xeno/Pred hybrid can do to wreck stuff, I wouldn't be venturing into a Xenomorph nest without anything less than the very best equipment possible. Hell, they even die going up against regular humans. Either we've just been seeing the very worst Predators, or they're not nearly as good as they like to pretend.
My impression of human spaceship technology in the Alien/Predator universe (from what little I've seen, I'm far from an expert) indicates that it's pretty unimpressive even as far as sci-fi in general goes. Wake me when a Predator ship can take on, say, the Enterprisefrom Star Trek in a single shot and come out unscathed, and then we can discuss how effective they'd be against 40k ships.
Now, I see what you're saying about the Space Marine thing, but I'm not sure it would happen. Have the Predators shown any sign of being skilled genetic manipulators? Do they think that the Marines would just let them get away with the bodies/geneseeds of fellow Marines? And even if the Predators do manage to figure it out, remember the horrendous failure rate that humans have with the process, and they've had 10,000 years of practice. Predators strike me as a fairly small population, and they'd probably never get more than say, 100 Predator Marines (assuming they even get it to work, considering how different the species are).