I assume your hair types are different from hair styles, which are accessible in the slave customization tab. If so, mainmenu.scn is the file that dictates the GUI structure used by mainmenu.gd. It is not a text file, so you will need to use the Godot editor to change it directly. You can also use code to change the GUI structure, but that can be less intuitive to many people. The Constants mod and Improved Random Portraits mod both use code to add their GUI controls.