Platies are far more comfortable at cool water temps than most people give them credit for. Central America/Mexico is subtropical, and once you factor in the high elevations many xiphophorus are found at, you have a fish that is entirely adapted to the temperature ranges one would normally keep goldfish at indoors. They are indeed incompatible, but not for lack of overlapping temperature requirements.
+1
I've kept platies at 72 before without problems, which is fine for a goldfish. Make sure you have a TON of filtration on there, and you should be good to go. As mentioned before, they will be eaten if they can fit in the goldies mouth. I also reccomend that you have a large enough tank to accomade the goldfish as an adult, anything under 20g is unacceptable for all species.