dramatically overcrowded fry are! They may be crowded by, oh..., a factor of 20. Would you happen to have an extra 19 10-gallons or four 50-gallon breeders around? ;)
Crowding causes stress. Stress causes immune systems and even hormonal systems to get messed up. As long-time Freshwater and Marine Aquarium columnist Paul Speice has written (seemingly forever) stress kills! Or, evidently, maims.
Please don't breed the deformed fish, even though their problems may be environmental. In fact, given crowding issues, don't breed anybody. (Easier said than done.) ;)
And as they get larger, the Gambusia will bully the guppies even more. :(