In the "My take on stuff" modmod I defined those myth and stories buildings like this
<BuildingInfo>
<BuildingClass>
BUILDINGCLASS_AMPHIBIAN_MYTH</BuildingClass>
<Type>BUILDING_AMPHIBIAN_MYTH</Type>
<MapCategoryTypes>
<MapCategoryType>MAPCATEGORY_EARTH</MapCategoryType>
</MapCategoryTypes>
<Description>TXT_KEY_BUILDING_AMPHIBIAN_MYTH</Description>
<Civilopedia>TXT_KEY_BUILDING_ANIMAL_MYTH_PEDIA</Civilopedia>
<Strategy>TXT_KEY_BUILDING_ANIMAL_MYTH_STRATEGY</Strategy>
<!-- Prerequisites -->
<PrereqTech>TECH_TAXONOMY</PrereqTech>
<!-- Cost -->
<iCost>-1</iCost>
<!-- Local -->
<CommerceChanges>
<iCommerce>0</iCommerce>
<iCommerce>1</iCommerce>
<iCommerce>1</iCommerce>
</CommerceChanges>
<!-- Global -->
<FreeBuilding>
BUILDINGCLASS_AMPHIBIAN_MYTH</FreeBuilding>
<!-- Other -->
<iAsset>1</iAsset>
<Flavors>
<Flavor>
<FlavorType>FLAVOR_CULTURE</FlavorType>
<iFlavor>1</iFlavor>
</Flavor>
<Flavor>
<FlavorType>FLAVOR_SCIENCE</FlavorType>
<iFlavor>1</iFlavor>
</Flavor>
</Flavors>
<bNeverCapture>1</bNeverCapture>
<ArtDefineTag>ART_DEF_BUILDING_AMPHIBIAN_MYTH</ArtDefineTag>
</BuildingInfo>
A building giving itself as a free building to all cities seems to work just fine when I've played with this setup, and it effectively halves the amount of buildings by removing the need for the _EFFECT buildings.
The bNeverCapture = 1 means that it can't suddenly spread like a disease between different nations when conquering or trading cities.