A biscuit soldier was undamaged by a G4 pistol. Despite being under the barrage & force of Luffy's G4 gatling, only the shields of the biscuit soldiers were being damaged and they still had the power to march forward. Both instances the soldiers weren't powered up by CoA.Their durability was a little over the place, a lot were taken out easy yet Machvise survived being hurled into Birdcage at a great speed and birdcage was supposed to be razor sharp cutting through stone as it touched it so if his body can survive that...
Also this thing about Cracker, we don't know that only G4 can break it, there's a wide gap between Gear 3 and 4 who's to say there's not a power level between the two gears which would be able to break the biscuits as well. For all we know Doffy's awakened strings(which are also unlimited) could break them as well. Doffy could make a black knight to attack Crackers main body and use his strings to take out the biscuits soldiers, possibly. What if Pica makes a Golem as big as in DR, you think a punch from something like that wouldn't have comparable force to G4 and wouldn't break a biscuit?
I wonder what would happen if Sugar touched a biscuit soldier? Would it also turn into a toy?
Sugar was only shown turning organic beings into toys. The biscuit soldier aren't organic. They are just snacks so Sugar's power wouldn't work.
Your average OP character is alot tougher & more durable than stone. Arlong could crush a stone pillar with his hands. Luffy could smash stone pillars with his hands. Luffy punched Arlong numerous times and it didn't leave a scratch. Just an example.
I kind of doubt Doffy can break Cracker's armor. You need a combination of stronger haki+foce. Luffy said Cracker's CoA was stronger than anyone he's ever seen and that includes Doflamingo, who's haki matched Luffy's. It took G4, which has A LOT of force that even Doflamingo couldn't fully block to smash the armor.