Did you saw what KN4 Naruto did? And KN8 broke through Chibaku Tensei also it has been implied that Kyuubi is half of the entire Juubi not only 1/9.
Remember it doesnt matter the order in Gedou Mazo, the only requirement is that Kyuubi is sealed last and according to Pain was just to balance the chakra of Kyuubi. In other words you need the 8 beasts just to balance the chakra of Kyuubi.
Hachibi and Kyuubi should win, all other Bijuus have elements like sand etc. Hachibi and Kyuubi specially Kyuubi are raw chakra.