you need to use blank reference files in the core mod.
A race that is not loaded needs the player entity but blank out as much as possible and make not select-able.
UI_FED-Units.brushes Core Mod
UI_FED-Units.brushes in the Add-on
pixelBox [ 2 , 333 , 167 , 75 ]
Keep all their buttons and textures on separate sheets, and call a blank brush file for the core mod. PlayerPics and Themes the same, files in the add-ons call the proper textures, but the core mod with no race loaded calls empty themes and pics and brush files.
The files all need to be in the manifests of the core mod but can be counts 0 and not call models and textures, let the add-ons handle that.