You can choose to plant or not, rkilling1 is quite correct, 3 guppies will pretty much be a max for a 5 gal. Esp if they breed, then you will need more space for fry. A suggestion, check for a cheap 10 gal tank, your stocking is larger and the tank is easier to take care of than a 5 gal. It doesn't take much more room than a 5 and most of the time the cost difference is minimal.
For guppies I prefer temps be at 78-80.