Someone else asked about plastic tanks and heaters last week. The general opinion seemed to be that the heater won't cause a problem if it isn't making contact with your tank. As long as it's mounted with some space around it, you'll be fine. I use a heater in my plastic hospital tank with no trouble.
Your light might do the job, too. My daughter's 2.5 gallon plastic tank has never had a heater in it. Her room is fairly warm and the light helps warm the water. It holds steady at 76 F. Drop a thermometer in and watch it for a few days. You might not need the heater.
Just make sure that at night, when the light is off, the temperature doesn't drop too much. If the temperature changes more than 3 degrees when you turn the light on/ off, it will be stressing your fish.
Someone told me that covering your tank with a blanket at night would make the tank retain its temperature.
