When I was writing the game I kind of imagined 4-6 riders and however many NPCs the crew wants in the Wagon Vehicle (mechanics, laborers, maybe a veterinarian or whatever.) I was kinda basing those numbers around two things: 1) traditional RPG party size and 2) actual cattle drive size.
Historically cattle drives needed about 10-12 cowboys. So I kinda scaled everything down. There are Fewer Kaiju in a drive than actual cattle in a historical cattle drive so fewer riders than cowboys.
You pretty much nailed the purpose of Kaiju Riding as a skill. I kinda imagined it as climbing up onto a wild Kaiju as a way to get your riders into a "bucking bronco" type situation which I feel is essential for a western style narrative. Clarification/errata might be in order for that skill.