I'd have to disagree about the MG06X here...it has effects, and as a rule you're better off using higher-quality outboard effects or doing your processing in your DAW. Also, it's missing the big draw for the Yamaha mixers: the one-knob compressor. Normally, these are useless for music...but for voiceover, now, that's different. You'll want your voice levels to stay in place against your background...and while the real device needed here is a ducker, compression on the voiceover goes some of the way to fixing your mix. Instead of the MG06X, I'd suggest the MG10...it doesn't have the onboard effects, but like I said, you're better off using something else, plus great rack processors are dimes on the dollar right now on the used market. And the '10 DOES have the compressors on two inputs for voiceovers, plus it's only $40 more than the 06.
Now, how to implement this...step one, get a decent ducker. Again, cheap rackmount stuff on the likes of Reverb or eBay are your best bets here, and something like a Symetrix 522 would run you $100-125. You'd sidechain this from your AUX off of the mic ins, and this would be able to lower your music bed if you run it through the ducker before the mixer. Perfect voiceovers! Then, if you want to add a bit more dynamic control, you could send your stereo output through a second stereo comp/limiter and smooth out the dynamics further, which also works nicely to keep your sound for picture nice and hot while not getting too punchy.