A common adage that I've heard when it comes to mixed species is: It works until it doesn't. In other words there are so many factors that come into play, especially with large carnivores. You may get a pair that live great together for years... until the day you find coyote bits scattered all over the yard. I've seen octopus that live just fine there whole life with rock cod... and then the next octopus promptly eats the cod.
Of course there is the old "raise them as pups together" trick...
As for fish in the bear stream, I wouldn't think it would hurt and probably provide an element of enrichment for the bears. That said I do remember the San Diego Zoo put live salmon in their polar bear exhibit and one of their bears got sick and died from eating the salmon (correct me if i'm wrong on that).