Hello!
Most of the time I would recommend FBX, since thats what I test out the most, but obj and gltf are also supported.
Principled BSDF works, but will be limited to Unitys standard-shader and can't have any custom shading. If you want custom shading, etc. a VRM file is recommended.
If you open a support ticket on the discord, there are people that make Assets for T.I.T.S. regularly and can give advice on everything related to the model creation ^^