The idea of adding a ninja unit, I think many of might like, but...
Ninjas are originally from and most associated with Japan, not China.
You might want to correct the empire.
The scarf idea, everyone is mentioning, is good.
If the pink sash or obi, changes color with the empire (aka, red for Japan), then, it should look pretty cool.
The ninja's weapon should Not be a katana. That is the Samurai unit's weapon.
Ninjas used shurikens or a kusarigama and thus would be the most appropiate weapon.
The black artwork from the neck down, looks cool, like what most of us would envision a Ninja might look like. Probably, from some bad movie though, lol.
The Chinese could have Shaolin monks or something. They don't need to steal a unit from Japan.
I saw a Chinese rocketeer in here, based on the Cho-ku-nu. That would be another example.
Just some appropiate constructive criticism.