Sure thing

I will allow it as long as you credit my name in your mod.

Can I use your mod in mine?

Thanks for your suggestion. Now a priority of adding Japanese clothing and armor is not high, but I will consider to contact with that development team when I needed.

If you're looking for Japanese clothing and armor, it might be a good idea to hit up the devs from the After The End Fan Fork mod. They've created assets for samurai armor + helmets, as well as kimonos IIRC. Even if the mod's halting development I'm sure the devs would let you use the assets.

This mod doesn't work with Umbra spherae and Umbra Spherae-Reborn, as Japanese culture in these mods use the portrait of Chinese (chinesegfx).

Could this work with Umbrae Sphera?

This mod lacks Yoroi (Japanese armor), Kabuto (helms), and portraits for Ryukyuan and Ainu people. (All of these are already included in the NMIH mod.)
As creating alternative data for them is a painstaking task, I'm not going make this mod enough to work with NMIH. For the same reason, I have no plans to suggest replacing the default portraits to the author of NMIH.

Would this mod work with the NMIH mod or is it for the base game only? I want to replace the default portraits for NMIH