Cavalry
There are four types of cavalry, five, if you count Mounted Infantry:
Ultra Heavy Cavalry - Basically, Renaissance Men at Arms in Gothic armour on top of a very large warhorse.
Heavy Cavalry - The rather more romantic version which hovers between plate mail and plate armour, and a happier looking steed.
Medium Cavalry - Usually the preserve of Sergeants, Squires and the more poorly equipped Knights; since the destriers are geldings, they don't care.
Light Cavalry - Minimum to no armour, and friskier horses.
With the best will in the world, Japanese Cavalry will never be described as Ultra Heavy, and charitably can be placed somewhere between Medium and Heavy. They use spears, not lances. What I'd like would be the equivalent of Dragon Princes, what we'll end up with is slightly below Silver Helms.
Whereas Bow Cavalry and Light Cavalry can be easily catered to, since both would go under the Fast Cavalry rule; however, YAri Cavalry and Katana Cavalry would expect to make a greater impact, especially if they were charging their opponents. This means that they'd need some form of upgrade from a special rule to provide for that impact, since a spear is not the equivalent of a lance (that's assuming the unit in question no longer qualifies for Fast Cavalry). Then you have to take account for the Oban unit.
The Naginata was added as a compromise, to compensate for the lack of a lance, and makes the unit more dangerous in prolonged combats; this weapon might be confined to the Hatamoto/Oban unit. Is this historically accurate? When you consider the halberd as options for mounted units in other factions, I'd have to say yes in this context.
Barding will be permitted for Heavy Cavalry.