they obeyed him because of his necklace and wood style
Then this necklace is like the Uchiha's genjutsu they use on the jinchuuriki. But as a fact is that the Kyuubi broke the necklace very easily. I dont think it could handle all 9 at the same time. And it was mentioned that Hashirama had all the jinchuurikis.
the necklace on its own was powerful enough to restrain 6-tailed kyuubi
and the necklace responds to hashirama's chakra so in hashirama's hands it will be more powerful.
necklace isn't like uchiha genjutsu
they obeyed him because of his necklace and wood style