1) I think you're going to have to further define what you mean by "rainbows" (also, please use commas). Like, boesemanni rainbows are far different from Pseudomugils. Pseudomugils might be shrimp safe, but I doubt a boesemanni will be. Pearl gourami would eat the smaller shrimp. And what other "nano fish" are you talking about? Pea puffers are technically nanos, but they're 100% not shrimp safe.
2) So here's info from a quick google search: a banana at 7-8" long (118 g) should have approximately 422 mg of potassium in it. Not including what potassium ends up being bioavailable or not.
Assuming you have a 10 gallon tank and you need to increase potassium by 2 mg/L , the 10 gallon tank = ~38 L, so you need 76 mg straight potassium, which translates to roughly (8"/422 mg * 76 mg = ) 1.44" of banana. Which, of course, doesn't include the sugars you're adding to the tank. About 1.5" of banana is still a lot of banana, and I'd be willing to bet it'd rot in your tank rather than provide any useful potassium.