All the goldfish strains you buy at the LFS are the same species, just different variations that have been selectivly bred. It's the same thing they do with dogs and other domestic animals.
It's a common myth IMo that goldfish require coldwater. In truth, when you look at where they are found native in the wild they are actually sub-tropical and can survive in a wide range of temp variations. They are very hardy and can be found introduced in a wide range of places and habitats around the world. You can find goldfish living and breeding places like Hawaii, Costa Rica, Columbia, Vietnam and other tropical places. Fishbase.org has a list of places different fish have been introduced, the one for goldfish is impressive. I certainly would not worry about keeping goldfish in the high 70's for the duration of their lives or higher for limited periods of time.
I cannot think of any reason why goldfish cannot be kept with some of the tropicals sold on the market except that it may look odd to have them together.