Honestly, Incas should not have access to the Xolotl Warrior after converting an enemy Stable.
This is not only because Xolotl is a Mesoamerican deity—making the unit thematically disconnected from the Andean world—but also because the Inca civilization had no cavalry tradition whatsoever. Allowing them to field a mounted unit, even through conversion mechanics, weakens both their historical identity and their intended gameplay niche.
One could argue that a South American cavalry unit might be a better thematic fit, such as a hypothetical Mapuche Bolas Rider. However, at this point, it would be more consistent and cleaner for the Incas to have no access to cavalry units at all, under any circumstances. Their design has always been centered on infantry and ranged warfare, and that identity should remain intact.
The Xolotl Warrior itself would make far more sense elsewhere. It could work well as a Unique Unit for a future Tlaxcaltec civilization, especially considering their alliance with the Spanish conquistadors and early exposure to horses. Alternatively, it could function as a shared regional unit for future Mesoamerican civilizations (Zapotecs, Tlaxcaltecs, Purépechas), reinforcing cultural cohesion without crossing regional boundaries.
That said, with the Slinger being turned into a regional unit, it would be reasonable to compensate the Incas with another form of exclusivity. A strong option would be an Elite or Imperial Slinger upgrade available only to the Incas, and not to other civilizations such as the Tupi, Muisca, or Mapuche. The base Slinger could use an alternative 3D model for those civilizations, while the Imperial Slinger would retain the current model, featuring the Hualcana (Inca shield), reinforcing its clear Inca identity.
This solution preserves historical coherence, maintains strong civilization identity, and avoids unnecessary overlap in regional unit design.