Can a 12 PRO charger be used on a 13 PRO?

Yes, you can use any Apple Charging Block to charge your iPhone, as long as the cable you use is compatible with both ports.

Yes. All Apple chargers are compatible with all Apple phones. An 18W 12 Pro charger will charge a 13 Pro; the recommended charger for the 13 Pro is 20 watts, not much difference at all. Perhaps a few minutes slower.