I wouldn't trust those other fish with silver dollars. Bichirs and peacock eels are predatory piscavores (fish eaters), and Oscars are both predatory and aggressive.
However, if you're commited to this experiment, I'm not saying it's impossible. I've seen silver dollars with predatory fishes in very large tanks. A 125-gallon tank isn't huge, but it might be large enough. The key is having the silver dollars be too big to fit in the mouths of any of the other fish. It helps that their disk-shaped bodies are harder for fish to get their mouths around. Full-grown silver dollars, which are at least six inches in diameter, are probably large enough, but juveniles, which is what are usually offered for sale, might not be. It would also help to have a full school instead of only a few. Therefore, I would suggest about 10 or 12 silver dollars in that tank. With a 125-gallon tank, your limit is not really the size of the tank, it's the tankmates you have in store for the silver dollars.
