Wait. Hold the Phone. Bruticus can scramble!? He's not just a glorious PCC!?
Now I want him more.
And I'm still convinced that a good portion of that combined super-weapon can be attached to his back, so he retains his twin back cannons, and so Onslaught's head isn't staring out of his back. There's a bunch of 5mm pegs on the bottom of Onslaught's weapon; it must clip on somewhere! It might also clip into Onslaught's vehicle mode roof, so he doesn't have that huge hold AND so he looks more like his in-game model.
Maybe Blast-Off's blasters can combine with the tips of the Onslaught weapon, to increase the length, and make them a better size match for Bruticus. Heck, the gun formed from combining Swindle's and Brawl's weapons make a rifle that's plenty big enough for Bruticus, and less silly looking then the super-weapon. And Vortex's over-sized swords would make good knifes or machetes for the big Brute.
For all we know, the "super-weapon" was created by the photographer, who was probably given no instructions, and just put the weapons together in a way that they all attached. From there on, it just stuck.
Look RIGHT next to the tires on his shoulders, on the plastic bordering Onslaught's head. Those are 5mm ports. The weapon goes there to close the gap!