I wonder how heir-designate would fit into this. I think that the heir-designate would not be viable to start a new Side. This could be why Jetstone popped so many Princes. One to assume the mantle of King (should anything happen), and the others to spin off new Sides once the diminishing Smuckers point is reached.
Parental consent may be a factor in spinning off a new Side, since only potential heirs actually have parents. For example, once Wanda popped, she could have been made heir-designate, which freed up Tommy to found a new Side once they had taken over a capitol site. Not necessarily Royal, but the Firebaughs do have parents and siblings.
This does bring up the issue of if GK would be able to do so, since they have not popped any potential heirs. Although, that would in turn explain why they haven't done so once they reached the diminishing Smuckers point, and already had a secondary capitol site.