My mom keeps her two bullfrogs in a cage she made by sinking a 20-high into one end of a coffee table and using plexiglass and wood molding to build a terrarium out of the table. She painted silicone onto the table and in the corners to seal it and there's some dirt/mulch and plants in there and the tank is filled with water for the frogs to swim. So you could do something similar but you have to scale it to the size of your frogs/tanks. You could also put some small fish in there but I'd suggest using only minnows like Gambusia or killis or something from a bait shop.
And I'd be hesitant to keep a turtle in a tank not because it would eat the fish and frogs (although it would which is a good reason to get minnows) but because of the filtration required. If you're keeping native turtles, it would be a good idea to keep them outside (I use kiddie pools which are really easy to clean).
But if you want to keep turtles inside, then yeah mud/musk turtles are a good choice because they stay small. Spotted turtles are also very small (the smallest turtles in the world if I'm not mistaked) but they're rare and heavily protected.
