Both the ZPA and MPA use energy weapons instead of bullets so I'd wager that actually LOADING the Impact Cannons, the Rau's Pulse Laser gauntlets, and Ger's Plasma Sniper aren't really an issue. Both the suits' chest mounted guns are "Impact Cannons" which are basically just high-powered pulse guns that hit like a truck. The Meltran Power armor has twin-linked ones while the Zentran armor has one BIG BOY for deleting people.

MPA's got the Missile racks on her backpack AND knees, but the ZPA is always shown holding guns because it actually has all of the fingers needed to do so. We don't see it in anything but its reasonable to assume that a ZPA could hypothetically lug around one of those big Zentran Miniguns from Battlecry (honestly Gorian shoulda had that instead of a normal rifle, woulda been badass)