If you haven't blown them yet.lol...just kidding.
Speaker wire, i use 12 gauge for 12 foot runs,i wouldn't go lighter.
Thicker is better if your going to go more than 15'
The QSC-MX series have balanced/unbalanced imputs,
The Fast Track Pro also has balanced/unbalanced outs, so i would use 1/4" balanced TRS cables from the interface to the amp if you can.
If you can't,keep the unbalanced cables short.(3 feet if you can.)
Put the switch on the back of the amp on parallel.
Don't run the amp to high,its better to have a good signal from the interface going to the amp rather than a weak imput signal and the amp cranked up to 2 oclock.
Keep an eye on the clip leds on the amp,you don't want those to light.As long as they don't light when your volume control on the interface is at the maximum volume you will ever listen at,your good.
If they do,bring down the maximum output levels from the interface and increase the amp volume output instead.
Balancing that will give you proper gain staging.
Hope that helps.