How does that apply to pokemon that evolve more then once? For example, if I wound up soloing Roxan with Torchic, would I be allowed to evolve it fully, or would I only be allowed to evolve one stage, and need to solo another Gym Leader with it to be able to evolve it again?
Also, may I aply this rule to Members of the Elite Four?
Yes, your Torchic would be able to fully evolve. If this were HG/SS, I would say no, but there are only so many gym leaders. And sure, you can count the E4.