Technically the only chakras to be "mixed" were the 2 halves of the kyuubi. Minato and Naruto didn't actually mix chakras, they "linked" them, which is a little different. I don't know why you would argue with SD over this issue when THIS PAGE
illustrates in a fairly easy to understand way how the process works.
To clarify (and put this issue to rest), Naruto's connection to the rest of the shinobi alliance (via chakra cloak) is similar to an electrical circuit that runs through your house powering the various appliances that plug into it. By fist bumping with Naruto, Minato was able to access that circuitry (like an electrical generator during a power outage) and send his own power through that circuitry.
When I read this I for one was completely astounded. If you think of it from the opposite direction it's even more revealing.
It's not just Naruto. It's Naruto, Minato, and the fully complete version of the kyuubi combined that Hashirama was comparing to his own chakra. Meaning OMG levels of chakra that astound enemies and allies alike are what Hashirama walks around with on a daily basis like its nothing special.
I think it's clear from this who the strongest hokage of all time is. (HINT: He doesn't have blonde hair.....har har.)