The main point is that commons, comets and shubunkins should really be in a pond, thoguh its ok to have them in an aquarium for the start of their life. I used to keep tropicals and goldfish together(didn't know any better, i was 9), but yeah the common goldfish ate everything.
Fancy goldfish can be kept with some 'tropical' fish, but only really sub tropical fish. You can have white clouds and zebra danios for example, but watch out because sometimes the zebra danios will nip fins, you should keep the temperature around 23 degrees celcius. Some types of corys do well in cooler water temps, but there are divided opinions about this so only try after lots of research and understanding. A heater should be used at all times too.