New versions of PhotoLab never overlay previous versions They need to be uninstalled separately. If you are happy with your installation of PL 6 than it is certainly OK to uninstall PL5. I only keep previous version available for comparison purposes when some feature in the latest version has been updated or is acting strangely. Generally, after several weeks with the latest version I uninstall it. Some people keep previous versions installed.
Pl 5 still installed because that is how it should be
instilling PL6 doesn’t remove PL 5 it’s ok for it to be this way.
For the time being I suggest live PL5 installed also if you can and wait for more response on your second question.
It is certainly safe to uninstall previous versions. Though some leave them installed, I personally remove them after insuring that the new version is stable and working properly, they are taking up valuable disc space and version N will almost always do what version N-1 will do, most often better. Just be sure to keep the installation files for the older versions in case you ever wish to reinstall them.