Can I delete MacFamilyTree 8, when I have MacFamilyTree 9 installed?

Yes, if you are using MacFamilyTree 9 there is no reason to keep your installation of MacFamilyTree 8.You can just delete the file "MacFamilyTree 8.x" in the "Applications" folder. Deleting MacFamilyTree 8 will not delete any databases, as the databases are stored in a separate folder.