WHAT!!!! that is lunacy? They would be smashed under the weight of iron think what you are saying, iron hands took on 4 legions on the drop site massacre and were winning. Now you add the iron warriors. that want nothing more to crush anyone that thinks, that they cane not be touched in their fortress the Fists and Children would be wiped off the face of the battlefield.
Why dont you actually put reasons why you actually believe that the Emperor's Children and the Mustard Marines could stand a chance, such as tactics, weaponry, planning, booby traps, the whole 9 yards, etc.
damn this is a hard question i do love entrechment of the iron warrions and the imperial fists but i also like to robotics of the iron hands i think the iron hands with there biotics might be a boost to maybe win but last time fulgrim and ferrist fought it dint end well for the iron hands though the fulgrim was boosted by the demon inside his sword and it was only after he started the hersey did it posses him. i think i might be a stale mate