Can I delete MacFamilyTree 7, when I have MacFamilyTree 8 installed?

Yes, if you are using MacFamilyTree 8 there is no reason to keep your installation of MacFamilyTree 7.You can just delete the file "MacFamilyTree 7.x" in the "Applications" folder. Deleting MacFamilyTree 6 will not delete any databases, as the databases are stored in a separate folder.