I've already used these linear tactics age line and light infantry for the Koreans and Japanese. In my mod there are Line Infantry (Musketman upgrade), Light Infantry/Skirmisher (Longbowman and Man at Arms upgrade), Dragoons (medium cav), Light Horse (light cav) and Cuirassier (heavy cav) available at Military Tradition/Science. As you can guess finding cavalry units suitable for the Japanese was very hard, let alone those with gunpowder attacks. Koreans can share units with Chinese and Mongols fine most of the time. But the Japanese have such distinct uniforms/armor that settling for Qing units shouldn't satisfy historically minded modders. But for now that will have to do. Using western units for the Japanese at around Nationalism (post Boshin War) is actually appropriate as advised by French instructors, they were dressed and armed like French and US troops. Ironically, this is where people have made units for them.
The Firelancer is dressed in a non Japanese far eastern manner and would look at home serving the mongols, chinese and koreans. I use it as a Gunpowder tech cruise missile unit for all civs as gunpowder specialists at that period were often hired/coerced from the northern chinese.