OK - First things first. The easiest and most efficient way to use your gear is to bring two speakers and plug them into one channel each of the amp. You can feature stereo this way, or use mono by keeping the channel pan controls at center. For stereo - use two channels per signal and pan hard left and hard right.
Two speakers with 400watts is plenty for 200-400 people if it's not a "bass intense" program. If you do a lot of heavy dance stuff ... you mat want to beef up the baottom end a bit, and you can use the QSC and an extra cabinet for that.
Take an aux send from the mixer ..... send it to the QSC input and use the BRIDGE outout to power ONE 8 ohm speaker. Now, you have a separate control to send any intrument or mic to that bass speaker independantly of the main mix. Works like a poor man's crossover.

Check the manual ... I think the mixer has dedicated outs for a mono send. Too bad the amps don't have a dedicated INPUT jack. That was for the easiest, safest operation.

I advise ... Use the 2 Fender speakers (one each to A & B )and keep the QSC as a spare od for monitors or extra bass. That system will kick butt as is.