Yes there are a few 6-seaters in addition to the usual 4-seaters. They cannot be reserved separately. You just reserve "a cart".
You have to ask for a 6-seater if you want one when you pick up your cart. They are about $20 a day more. If they have one they will swap out your 4-seater for the 6- and charge you the difference. If they don't have one, you can be put on a waiting list in case one becomes available during your stay.
The carts have a retractable plug and most campsites have a power plug on a post out near your loop's road to plug into for charging.
Bama Ed