Naruto and Killer Bee bested 5 Tailed Beasts, not Kurama or Gyuki. Plus the fact the 5 Tailed Beasts were under Obito's control, who couldn't make proper use of their powers. As later stated in the manga, Jinchuriki is far more powerful than the Tailed Beast itself. Kurama by itself got owned by SM Naruto, so clearly it's not stronger than 8 Tailed Beasts.
Naruto gets raped low difficulty. Killer Bee would push him high difficulty, let alone all 8 Jinchuriki together. 2 Jinchuriki are enough.