I honestly wonder how such extreme dimorphism between the bull and neotenic males could remain viable during genetic mixing, you'd probably end up with scrawny bulls that never make it beyond their teenage development or something.
What would the resultant females be, too?
They seem sorta specialized enough that mixing could also result in a weaker hybrid.
You could probably pull off your idea without hybridizing or mixing the species, just doing a more conventional R/P/S like what's found on earth. The bobbysoxer by itself could pull this off, and given the females lose their tusks you could make males that mimic females to infiltrate harems, monogamous weak tusked males, and harem forming strong tusked males your game.
Giving them behaviors that woo the ladies, and also childrearing aid, would make this even better since that would mean the mimics can woo while in the harem, females would be happy with the weak tusk males that help with the kids, and the harem males are just... the usual.