Guys, Ryou is not "advanced" armament Haki. I've made the same mistake before. Ryou is simply another name for Armamenr overall, like Mantra was for Observation Haki. However, because Wano specializes in Samurai, Haki was taught as a "flow" rather than hardening (this was Hyougoros whole explanation in Udon), more akin to Zoro cutting what he wanted to cut in Alabasta.
So in reality, most of you are talking about the 2nd and 3rd levels of Armament Haki (the Shockwave and the "flow" version), but all 3 levels are "Ryou".
Anyway, the way Kaido describes it, it seems like all 9 scabbards were needed to truly produce this 3rd level of Armament. Kiku pierced Kaidos hand not because Kiku had that Haki on her own, but because all 9 "produced" Oden's haki together, like a single being. Same with everyone else who managed to hurt him.
I thought I'd just point out the difference because I feel like people are misunderstanding what actually happened.
Also, it was extremely clear that WB did not have this "tough skin" power. Random, nameless marine fodder were able to stab him, shoot him and make him bleed in Marineford. To assume Akainu could one shot Kaido, when very clearly Kaido would not be damaged the same way as these nameless fodder marines damaged WB, is kind of weird. In fact, the Red Scabbards damage to Kaido seemed like it did nothing at the end of the chapter.