This page gives some basic guidelines to follow when mixing different types of goldfish
Many goldfish species have been carefully bred to produce some amazing results. The goldfish with flowing tails are much slower swimmers than the comet goldfish. In a competitive environment the comet goldfish will outmaneuver and bully the slower fancy-tailed goldfish. This can cause many problems at feeding time, you may end up overfeeding your comets and underfeeding your others. Similarly, telescopic eyed goldfish or bubble-eyed goldfish have a much more difficult time tracking down their food than even the veil tails or fan tails and again, will be less able to defend themselves. It's important that you try to closely monitor the fish for any possible issues like these and separate them if possible. And always always always make sure that none of your fish could fit inside the mouths of the other fish.