I tend to keep a variety of fish and critters in my pond and stray away from just goldish and koi.
One of my ponds has a pair of blue gill, yellow perch and a variety of dace and darters. No koi or goldfish in there. Have 2 3000gph pumps feeding into the waterfall, so it stays well oxygenated. The male blue gill hovers over an old pot filled with pea gravel. Outside of the blue gill, only way to tell there are other fish in there is during feeding time.
Main large pond has two terrapins in it, a juvenile red ear slider, with mostly goldfish, orfes, and a few koi. Never messed with keeping bass as they tend to eat a lot (as in other fish) and get rather large. If my pond stayed cool enough during the summer, I would probably get a sterlet, but I know it gets to warm.