I wouldn't worry about them not eating fish right away. Some turtles actually don't really care for them, and I think the instinct is to go after food that moves and it doesn't have to be fish. (They will eat them in the wild, but not as much as is commonly assumed.) At some point they'll likely become interested in them. If not, they've got "friends."
About the back claw that's missing--How many claws does your turtle have? If it's four and looks like one is missing (check the other hind foot), that's pretty much normal. One nail is typically underdeveloped or not developed.
If your turtle has 3 hind claws on the foot, he could have lost one trying to climb up something or otherwise gotten it caught. Is his basking area easy to get on? Is there something else in the tank he could have caught the nail on?
"You become responsible, forever, for what you have tamed." -Antoine de Saint Exupery-