I was at my LFS and saw a pair of African Goliath Frogs for sale. It sparked my curiosity and got me wondering if they can be kept with turtles and/or large fish?
First off, are you sure it was a true Goliath Frog (Conraua goliath)? These frogs are endangered and only a few dozen are exported from their home country of Cameroon every year. Most likely they're Conraua alleni, a very similar looking relative.
Either goliath cannot be trusted with anything it can fit in it's mouth. Small note: their croak can be heard a mile away. And, they croak predominantly at dusk and dawn. Do you really want that sort of sleep deprivation?