The current/voltage on the probes will not be big enough to cause any problems. but the current is not what ignites fuel its the spark.
the gauge is just to be used as a guide, not a measuring device. when the level gets down close to the tip of the probe it will yes, it will be inconsistent but you know its close to the bottom and the more the light comes on the closer to the bottom it is.
The only other way is to use a flow meter but i havent seen anything that small to fit on a rc car.