I doubt the Corid is the culprit. If they were molting during dosing, maybe, but it shouldn't affect existing feathers any more than a person's straight hair would suddenly become curly after taking medication. It can only affect growth that comes after, not before.
I also have one girl who has a somewhat similar tail. I thought she would molt it out, but her molt was extremely light and her tail feathers were never lost. I think it's more due to wear and tear, such as how they're sleeping, or a new place they found to hang out in. As long as they're otherwise healthy, I'm not sure how worried I'd be.
As a side note, I also try hard to avoid potentially harmful ingredients, and feed moistened Scratch and Peck grower to my chickens. However, sometimes the benefits of something outweigh that one ingredient that may be harmful. I try to keep them (and myself) healthy, but I also try to balance good vs bad. Giving something a few times will likely not result in catastrophe.