Yes, you can use the old Apple 5W charger to charge your iPhone 12 Pro Max. Of course it is the slowest charger out there and will take the longest time to fully charge your iPhone.
No, there will be no issues if you used the 20W Fast Charger from Apple. You can use it all day and there will be no issues. Each iPhone has a protective charging circuit which will prevent over-charging the iPhone including controlling the the flow of total Watts throughout the charging process. There is no truth to what you have heard people say in your second question.
Axel F.