I would recommend sticking with the 2 ohm subs. That amp doesn't appear to be 1 ohm steady (your choices with two 4 ohm subs is 1 ohm or 4 ohms and your choices for two 2 ohm subs is .5 ohms or 2 ohms).
No need for a capacitor. The big three is engine-to-chassis, alternator-to-battery positive...