He should honestly
Or at least
he should introduce the leaders like "Kondo/ Hijikata/ Okita" as being rogues and on the side of the alliance in this war (without the shinsingumi being formed yet)... maybe as a way to make them formed with the "New shogunate Momonosuke" as an example and they are his special police force... instead of Mimawarigumi which was working for Orochi and then sold themselves to Kaido
So at the very least, Oda honestly should at least introduce the individual leaders depictions
Yeah Shinsengumi was forces that protected Shogun in the past. I think scabbards might be based on Shinsengumi since they swore loyalty to Oden, former Shogun. There are also no female in scabbards.
I think Okiku might be inspired from Okita SOuji. Just like Okiku, Okita Souji was known as handsome. Okita sword was also Kiku Ichimonji. There's Kiku in Okita's sword name.
Oda also introduced Hanzo Hattori, famous ninja in history but his portrayal in One Piece manga is bad. I think in the past Hanzo might be skinny like Shinobu and might help Shinobu.
Hope Oda also introduce characters based on Kotaro Fuma and Sasuke Sarutobi too.