He probably knows by now, but Yonny doesn't spawn in a specific cave. It's just the amount of rescues you get. They don't want you to miss Yonny somewhere and never get him. So, yeah, they're in order of when Nintendo thinks you oughta have em.
Castaways are not based on location, they're based on a specific order. If you skip Last-Frost Cavern, you can still find Russ in the Crackling Cauldron.