There are a lot of fish which can live in cooler water in the tropical fish store, including most barbs, blind cave fish, red horse minnows, and lots of cichlids. But when you add in that they must fit in a 5 gallon aquarium, and be compatible with guppies....all that come to mind are bloodfins, white cloud mountain fish, and perhaps some Corydoras aeneus or paleatus to keep the bottom clean. I'm sure there are many more that I'm not thinking of....maybe some other readers can suggest some more. The fish I named would all prefer water about 70 or 72 degrees.