Looks like what you want is granted by a Leader trait so it should be easy:
Make a folder for the module: For example Assets/Modules/MBE. (MBE for "Multicultural Balseraph Empire".)
In the module's folder you'll need copies of CIV4LeaderHeadInfos.xml and CIV4CivilizationsSchema.xml from FFH2's Assets/XML/Civilizations folder.
Rename both files so that, say, "MBE_" begins each file name.
In the leaderheadinfos file (the one in the module folder) get rid of all the leader entries except the 2 Bals. leaders. From and including <LeaderHeadInfo> to </LeaderHeadInfo> for each non-Bals. leader.
EDIT: Oh yeah - in the Traits section of the add Tolerant, or change one of the present ones to Tolerant.
At the beginning of the file look for:
<Civ4LeaderHeadInfos xmlns="x-schema:CIV4CivilizationsSchema.xml">
Change CIV4CivilizationsSchema.xml to "MBE_CIV4CivilizationsSchema.xml
Enable modules in the Fall From Heaven 2 .ini file: "ModularLoading" - Change the "0" to a "1".