This is scifi, campy scifi, where rule of cool is all that matters. Practicality and effectiveness are non issues.
If you're looking for counter points though, take note this is power armor, it doesn't care about "man portable". The suit carries that gigantic drum of ammo you speak of, and it could easily computer stabilize the gun for perfect accuracy.
All the weapons you list are just other options, none better than the other, and all of them are available for this suit.