Guppies can survive in cold water, but you should find they'd healthier and would live longer if the water is slightly heated (around 76-78F). This could just mean putting the tank in a warm room, or adding a small heater.
Guppies do just fine with only one gender in the tank. You may find, however, that females you buy could already be pregnant. I'd go with an all-male tank, as they are usually brighter and prettier than females, and you have no chance of having any fry
Good luck with them, and welcome to the forums!!