Hello axemanchris,
Retrologue is part of the Cubase Set and installed in the common VST3 path, you should not have any problems with it. Groove Agent One can be copied from previous Cubase versions paths over to the common VST3 folder (or the specific CP8 folder). Indeed, it is also present in the Additional Content folder of the ISO images / full installers (Win).
As per third parties, the paths will be imported automatically - if not, you can open 7.5 Plug-in information, take note of the scanned paths and add them to CP8. But this should not be necessary.
Agree with what marQs said, never uninstall a previous version which is working fine before you are 110% sure everything works more than fine on the new version as well.
Best,