Should I delete an old Photo library having created a new one?
It's unlikely that there will be any conflict as Photos will pick the new one by default. Nevertheless you will gain some disk space back if you remove the unused library.
To be on the safe side

Verify that all the photos are in the new library in full quality
Create a backup of the old library on an external device
Rename the old library, reboot and keep using the system for 1-2 weeks
If everything works as expected, delete the old library (but keep the backup around for a while)