Depends on what your room temperature is. Like if you live in Thailand, it's going to be warmer then in Alaska. Usually it's not a good idea though, because of temperature swings which can stress them out.
Ya. I'd use a heater, as I live in Minnesota. It'll be nice and sunny out one minute and sooner than you know it, it'll rain, thunder and lighting. I'm not kidding.
I know people who keep their Endlers at 70F which is cool for a tropical tank and as long as they were acclimatised slowly to this temp they should be ok and so should guppies. However fry will grow much more quickly as a more normal temp of 76-78 ish.
I would use a heater and set it to 74-76F ( but check with a thermometer that the heater is actually giving the same temp as its set at), and see how they do.
I completely cover my tanks with a piece doubled bubble wrap at night. I have 8 tanks and when I go in in the morning only one or two heaters are showing that they are on. If I forget to bubble wrap then most of them are on. So it really does save electicity to wrap the tanks up at night if you dont already.