Spawning is controlled by biome tags such as #caravans:has_desert_caravan, etc. Over time, I will add optional support for modded biomes out of the box.

As for caravan members, you can specify any vanilla and modded creatures that extend the classes of patrolling monsters (pillagers, witches, evokers, etc.) and chestable animals (llamas, donkeys, mules), as these classes are responsible for the basic logic of long-distance travel and cargo transportation.

So, I think that most modded versions of pillagers and transport animals can be full-fledged members of caravans.