Naruto has 4 times kakashi (average) level chakra, after taking Kurama chakra he uses it as his own. I don't know how you concluded his own mixes with Kurama senjutsu? Yes relative to Madara and Hashirama his own reserves are negligible. It took Madara 24 hours battling to run out of chakra to the point that his back would touch the floor. And he wasn't completely out of chakra either.Naruto having enough chakra to mix with Kurama's senjutsu is "small"