There seem to be whole websites dedicated just to the temperature range in which guppies can live.
They do seem to be happy in the same temperature range (75-85) as all the other fish in the Freshwater tank, compared to the 65-70 of goldfish.
http://home.clara.net/xenotoca/temperat.htm
Not trying to challenge anything, just that, realistically, they would thrive in the Freshwater Aquarium.