id see how the servo goes at 7.2v before cranking it up.
As for the Stampede, I’d go for a system that allows 2S now and be fast but could handle 3s if you want to go nuts.... short grass isn’t an issue but the long stuff will heat things up a bit. Running through puddles is the same. These motors have instant max torque, more resistance more heat and load on motor and ESC.