I don't have a scientific or definitive answer either. I do have a FG pool and have "shocked" up to 25ppm once without any adverse affects, other than staining. This is supposedly due to metals in the water however I have my doubts since everyone who has tested my water says there aren't any metals. I haven't figured it out yet but it cannot be caused solely by the High FC level since I generally have to do an "ascorbic acid stain treatment" a few times each season and rarely have to shock. My pool color is the same as Waterbears, see his picture in the upper left hand corner of his post. I also know someone with a white pool and they use dichlor to shock their pool. No telling what level they've gone up to as I have seen numerous empty bags around after they opened. Back to your original question, I can't imagine a FC level of say 50ppm hurting the fiberglass. As an avid boater, I have used some pretty nasty and acidic stuff on my boat without any adverse effects. I would venture a guess that at some point, chlorine (or acid for that matter) could cause the gel coat to oxidize (develope a white haze on the top layer). What that point would be I haven't a clue. Let me stress that this is only a theory.

Dave