The voltage rating of the capacitor is just how much it maximum can handle. Even if you want to use a 100v capacitor on just 8.4v no problem but it will be bigger. If you want to go small just use a voltage just above the battery you will use although with 2S batteries I would use 16v instead of 10v capacitors. Tolerances are wide with capacitors so it is not wise to sit close on the limits.