Foreword: To moderators, If this is in the wrong section, my apologies, I am quite new to Total War and the forum. I will happily move to a new section
If you do not want to read what became an essay, skip to the bottom.
After hunting bookstores for resources on samurai warfare, I have come to the same conclusion many in the community have already reached. The unit types are historically off. Formations of katana samurai/ mounted katana samurai did not exist, as the katana is a side weapon or, more rarely, used in an honor duel (I'll go into details with this if requested).
The vast, absolute vast majority of early to mid Sengoku period Samurai "on foot" carried the yari. Other foot samurai of this approximate time frame carried the nodachi (seriously limited use) the naginata (limited use) the bow (which was a job moving to the ashigaru) and various clubs and bludgeons (like the Tambo, and again, limited use). All samurai carried the katana... as a backup, because quite frankly in a one on one fight the yari can outclass the katana... and yaris in formation? If you're a "katana samurai," just go home.
Towards the end of the end of the era samurai "on foot" were either using the matchlock and the yari or leading foot soldiers into battle. But even our beloved yari was under threat, as the experiences in the invasion of Korea showed, that the matchlock outclassed all and was the definite future.
Mounted samurai of the early- early mid period were multitasking "lancer calvary." They would never use the katana solely, only after they lost their yari in some poor ashigaru's chest. When they could they would use their bows to harass and then move in for a charge. There was no separation of roles. Rich samurai would have a servant follow or receive them to pick up a new yari or drop off his bow.
Towards the end of Sengoku, calvary maintain the skirmisher/ heavy shock troop roles. The bow was replaced with the musket and charges were less common due to the convenient abundance of pointy sticks
Ashigaru are more or less the most accurate of the three "classes" in the game. However I feel that muskets are introduced too late or gained through historically inaccurate means( Really using imported matchlock units will require a large portion of your population to become Christian. However Christianity did not spread like wildfire like it can in the game. Christianity only accounted for ~2% of the total population of japan in this time period)
Sooo lets get down to business
Realistic Samurai Units:
"On Foot"
-Yari Samurai (Obviously with better stats than it currently has)
-Bow Samurai
-(Maybe) Naginata Samurai
-(Maybe a shock unit) Nodachi or Tambo Samurai.... (Maybe)
-Matchlock Samurai
"Mounted"
-Light Calvary
-'All purpose' Yari Calvary that uses the bow.. but isn't amazing with it.. and the yari as well as the "Light Cav"
-'Shock Troops' Heavily armored with a big yari in hand
-Generals (pretty accurate in current state)
-Musket Calvary (Similar to the 'all purpose' but uses musket and isn't as effective in direct combat)