To be honest you're more likely to overheat him with a lamp.
If you get a heater that has a thermostat, it will automatically turn itself off once it reaches the temp it is set to. Then it will only turn back on if the temp gets lower than the setting.
I have heated my 2.5 gal quarantine tank with an adjustable heater that had a thermostat designed for a 10 gallon tank.. No problems whatsoever.
Personally I would not add anything else to a 2.5 gal. Definitely not a snail, they create way too much bioload, especially if your tank isn't cycled.