To use a heavy gun with a small body type, change to a heavy body type and select a gun (make sure it's exactly how you want it as you cannot edit it afterward). Choose any gun of any weight. Now, do the same for your secondary and make your default weapon. Go change back into a medium/light body type. Your default guns will stay the same, but your body will not, thus, when you play matches, you can have a light body type with a gatling gun.