But that means that he essentially has unlimited chakra and should just one shot the entire battlefield. Let me explain:
In Pt.1 (lol) Naruto had to have a necessary level of chakra to summon the giant toad. He did this by drawing chakra from the Nine Tails, using that energy instead of his own. This implies that you need X amount of chakra to perform a summoning. The smaller/weaker the summoning, the smaller x is. The bigger/stronger the summoning, the larger x is. If a ninja doesn't have the necessary chakra levels, the summoning won't happen. Like when the sand village attacked in the first part, and a bunch of ninja gathered together for a joint summoning. The individual didn't have the necessary energy to complete the summoning, so multiple ninja came together to meet the requirement.
Kabuto is summoning not just a couple of powerful Ninja, but literally an entire army of Jounin level + ninja. Several of which are some of the most powerful to ever exist. Orochimaru struggled to summon three, and failed at that (though due to outside circumstances). Kabuto magically 'surpasses' Orochimaru by not just outdoing his jutsu, but literally decimating it. He's summoned at least six kage level ninja. Not even going into all of the dead jounin, swordsman from the mist village, former Akatsuki members, and hosts. The amount of chakra necessary to pull this off is off the charts.
When I see a summoning in Naruto, I always figured it meant the character summoning it was just as powerful. If he wasn't, his chakra levels wouldn't be at the necessary point to pull off the summoning. So when this former Kage who is fighting Gaara and Naruto is pulling off all of the ludicrous feats, I'm seeing it as Kabuto being capable of the same level. Now multiply this by the number of former kages he has summoned.
If he has enough chakra to pull off a summoning like that, then he should be able to just unleash all of it and kill everyone at once or something. He would have to be inanely powerful.