Human chakra actually will be sufficient, it was stated that Raikage was bijuu-level chakra, i know hes particuarly strong, but if one guy can have that much chakra then imagine how much chakra thousands of humans could give to the GM. Besides, this would be only be a substitute until he can obtain the hachibi and kyuubi. Theres also the fact that the zetsus must be stealing peoples chakra for a reason.
Both scenarios are possible, in fact both scenarios may even be possible at the same time. He might just take human chakra in addition to the chakra of the Kin Gin Brothers.