Namely, when a PPCer recruits a character, they don't remain the exact same character. The badfic version and the PPC version share certain superficial traits, but the whole point, IMO, is to take the kernel of an idea that was done badly and show how it could be done well.
Ilraen, for instance, is not the same Andalite who showed up for one line in a fanfic to give some OC morphing powers. He did that thing, but that other guy was just a one-dimensional bit character without so much as a name. Ilraen is, I devoutly hope, much more than that. You couldn't say at this point that they're the same character.
Same goes for Derik. He had a bit more to his badfic origin, but Derik is not the same guy as E'rik, because he has an inner life with conflicts all his own. E'rik... doesn't. He was just there to look impressive and play backup to the main Sue of the story. Derik remembers being Weyrleader to Alanna's Weyrwoman... but he remembers a lot more about having been a harper and a generic bronzerider, none of which was actually in the fic; and he also has these strange moments where the fact that he's a Phantom photocopy seems to go further than skin-deep, and they freak him out.
Even Gall, who's the most similar to her badfic counterpart, I wouldn't say is the same character. Luga is a generic schoolyard bully who bullies for the sake of bullying with no real rhyme or reason to it. Gall is a self-centered jerk who likes to throw her weight around (such as it is), but she's also perceptive, insightful, and intelligent; she respects power, loves her dragon, and wants to jump her partner's bones if he'll ever take a hint. Her best friend is Gremlin, and they like to bond over raucous girls' nights and physical activities such as AHAIRQ and the PPC Hunger Games. They're different people.
At best, you could say these characters were inspired by their badfic counterparts. They share similar origins, but nothing more. And that, I think, is how you avoid plagiarism. Credit to the original not required.
~Neshomeh