if you want to create a unit that has a spaced name like "Gothic Knights" always put " to the names.
"...Program Files\SEGA\Medieval II Total War\armies"
...and open the .cax file in notepad look for...
"< UniqueType>Byzantine Infantry< /UniqueType>" (without spaces)
...and that will be the unit id of that particular type of soldier. The unit ID will not always match the name in the game.