Can I delete an older version of MacFamilyTree, when I have MacFamilyTree 10 installed?

Yes, if you are using MacFamilyTree 10 there is no reason to keep your installation of MacFamilyTree 9 or 8.You can just delete the file "MacFamilyTree 8.x" in the "Applications" folder. Deleting MacFamilyTree will not delete any databases, as the databases are stored in a separate folder.