Would a mixed exhibit with masu salmon, japanese giant salamander, barbel steed and whitespotted char work? I would have thought that the salamander would eat the fish but the giant salamanders at the Osaka Aquarium share a tank with some medium-sized freshwater fish (char I think, I'm not 100% sure though). Also american beavers with largemouth bass, black crappie, white crappie and american paddlefish?
First mix would not work simply because Japanese giant salamanders eat fish. I'd be wary of the second tank too, as I'm pretty sure the bass would eat the crappie if they could.