Cars are usually loaded onto a Ro-ro on their own power, and will have some amount of fuel on board. They typically want 10-25% full tanks to account for various contingencies, but I don't know how carefully that's monitored. Given the amount of vehicles in question, probably not that closely, though presumably manufacturers are equipped to be reasonably systematic about fuel fill.

Bilge pumps should be able to keep up with firefighting pumps. But, yes, the firefighting pumps won't be able to handle more than several cars on fire.