@ RomanSoldier9001
To be able to select the Mongols in Custom Battle you just have to alter a line in the descr_sm_factions file. The file is found in C:\Program Files\SEGA\Medieval II Total War\mods\Stainless_Steel_6\data and you must change this
into
and be sure to change any other faction into a NO in custom battle availability because there is a limit to how many faction can be played in custom battle. So for each faction you swap in you must swap one out. Hope thats clear if not just let me know