Water temperature is just one of the things that will stress out fish if it's not the correct temperature. We know that stressing fish have a lower immune system. While mixing them can be done, one or the other will suffer from stress, be more sickly, etc.
There are many other fish, which are oftentimes sold as tropicals that are actually cool/cold water fish... that can be successfully mixed with Goldfish or be kept in their own cool/cold water environment. Let us know if you want more info on those.