Madara fought Hashirama only at that point so the other Senjus fought other Uchihas. You're basing your reply on after the fight was over, it showed Madara and Hashirama powering up and Hashirama was always able to beat Madara so why would he need help? So Hashirama needed help when it was just Madara but didn't when it's Madara+Kurama?
We all know Kurama (100%) is stronger but the difference is being able to use that power which a wild Bijuu is said to not be able to do as well as a Jinchuuriki. Madara was commanding Kurama to attack but didn't show to use close to it's full power, a swing of it's tail is said to crumble mountains and start tsunamis yet he didn't use that power.