In Czech zoos, crocodilians are kept with fish often - it seems to work when fish are big enough (caimans with pacu and plecostomus in Brno, dwarf crocodiles with large cichlids and some catfish in Dvůr Králove), small enough (gharials with Danio in Prague) or have a lot of space (slender-snouted crocodiles with African cichlids in Ostrava, Philippine crocodiles with large assembly of Asian fishes in Pilsen). But I think that especially in the last case, some losses are inevitable.
Unlike carnivores, many other predators do not have a hunting instinct, but only hunt when they are hungry. Therefore, crocodiles (or sharks, for example) are often mixed with potential prey. Since they know they will be fed, this usually works without any problems.