Hello and welcome to the forum!
No, you don't need a heater. Your water temperature is at 26C. (Which is 78.8 degrees Fahrenheit)
I have to ask... what is your pH, GH, and KH? You can find this information out by going to your local water providers website, or by calling them. We need actual numbers. (Ex. ppm)
Guppies require hard water and a lot of space to swim. If you have 2 males, they will most likely act aggressive towards each other. Guppies should be kept in a 2:1 ratio. (2 males for every female)