Are the new 18W power adapters for the iPhone 11's compatible with the MacBook Pro 13-inch (2019)?
Is it safe? Anything to watch out for?
What about using my MacBook's adapter (with the correct cable) on my iPhone 11 Pro? Is that safe
Yes, it is safe to connect the 18W USB-C power adapter to a MacBook Pro. However, I wouldn't expect it to actually be useful for charging the laptop. The charger supports 5V (max 15W) and 9V (max 18W), but with the MacBook Pro you really want a 15V or 20V charger to be able to get some energy transferred in a timely manner.
And yes, it is also safe to use the MacBook Pro's USB-C charger with the iPhone 11 Pro using the correct cable. It should be able to charge your phone just fine.