Are gold fishes black?
Yes. Some are black. It depends on the genetics. Our Gold fish had babies, and they are all black exsept for a few.
You can definitely do that, but you haven't made it clear that you have cold water in the tank before. If you can keep the water temperature at 72 F, then you can keep both cold water fishes like gold fishes and non-aggressive community tropical fishes like gouramis, platys, mollies, algae eaters etc with gold fish.