Another point that Masashi was making with his fusion of Madara and Hashirama, was to answer the questions of how strong the Senju are. We see Masashi explicitly showing Madara using Hashirama’s techniques. I believe Masashi showing us this, is his way of telling us why the Senju were equaled to the Uchiha. And on a side note, unless Masashi shows the fight between them, I do not think Madara had the Perfect Susano’o whilst fighting Hashirama. Masashi, before Madara activated his Perfect Susano’o, gave us a close up of Madara’s Rinnegan. I would confidently say that Madara first unlock the Rinnegan after his fight with Hashirama, whilst made Madara have a near death experience (possibly a way to unlock the Rinnegan). That being said, I think the Rinnegan is required to perform the Perfect Susano’o, if so then Hashirama only had a fight with EMS Madara, not EMS + Rinnegan Madara.