I was making a mod with new units starting with the 2000. After I tested it I realized that monster #2013 up is used by EA Hinnom and other. I think there should be a warning about this in the modding pfd. There is the warning that "It mustn't be the same as any other monster" but I think it is very misleading since 2013,2014,2015,2016, up to 2082 are between 2000 and 2999 but they are being used by Other Default Nations.
6.2 #newmonster <monster nbr>
Monster nbr should be a number between 2000
and 2999. It mustn’t be the same as any other
monster. This new monster will be affected by the
following modding commands and there should
be an #end command at the end.