Indra is more powerful then Hagoromo and Hamura , nice I didn't know that . Half Hagoromo's power is stronger then full powered Hagoromo. You can watch this like 9 separate bijuus aganist juubi , because you can count susano like one bijuu.
Firstly, Indra's Arrow has nothing to do with Indra. It's Sasuke Uchiha's Ultimate Susanoo Lightning Arrow named as Indra's Arrow. So this has nothing to do with Indra being stronger than whatever.
Secondly, Hagoromo's powers are from the Juubi. That doesn't mean raw Juubi possesses the same power as its jinchuuriki. Even Hashirama owned Juubi. He could have never done that to "BijuuPS".
Thirdly, you got the relation wrong. PS can be counted as one Bijuu i.e. Kyuubi (jinchuuriki manifestation). But "BijuuPS" can be counted as Juubi since it's formed by absorbing all bijuus chakra and merging it to create the Juubi's chakra, and then combining the Juubi's chakra with Susanoo. The raw power output of that is much greater than pure Juubi which couldn't even break Hashirama's Gate of the Great God.
And to tell you one thing:
The power of Juubi =/= 9x the power of Bijuu.