No, a smaller pinion will not make the motor heat up 'more' than it already is. It will be easier on the motor, hence less heat. But just lowering the pinion isn't going to cut it.
The reason your ESC is heating up is because your motor is straining so hard to get the truck moving it is putting...
A 6900kv motor is not going to be good in a stadium truck. It won't have enough torque. You could go with a lot smaller pinion and a lot bigger spur if they are available, but I would replace the motor. Stock, the brushless VXL Rustler only has a 3500kv motor, which would be a more optimal kv...