I would certainly not recommend more gravel! With the amount of gravel you already have, you will already have problems keeping it clean and oxygenated. The problem with "gravel slope" setups is that the gravel at the bottom doesn't get enough water circulation, so it tends to become anaerobic (non-oxygenated) and this is bad. Also, whatever dirt falls into that gravel is impossible to clean out. As a long-term setup, gravel slopes should be avoided, IMHO.
Regardless of the heater, the essential questions for any newt/sal are... what is the max summer temp, and how cold can you get it if you remove the heater? For caudates, the issue is more of how to cool the tank, not how to heat it. If you are planning to include an animal that needs the heater, then forget about caudates.