Agreeing with Bay here. I mean, it might be interesting to see older characters learning to be Pokemon trainers. But with what you have currently, you have the plot hole of why do these people kidnap a band member? And why is the ransom the three Gerbits? If you remove the entire Pokemon part of the plot, then you're left with a mystery as to finding out who these people are and why they kidnapped a band mate, and there's also the thriller part of if the band mate can return safely.
If you want to include the Pokemon half of the plot, then you're going to have to answer the questions: Why don't the antagonists steal the Gerbits themselves? Why do they go with this plan? Why do they target the band specifically?