WP:UNIGUIDE says that they must have an article but that directly conflicts with WP:N so I ignore that part of the recommended guidelines.  I understand your reservations about not having a hard-and-fast rule but there aren't hard-and-fast rules governing the other sections of articles and things seem to work ok.  If there really are enough interesting alumni that the list grows too long then just split it out into a separate article.  And we can all object if someone adds one or more alumni that are clearly minor figures that shouldn't even be mentioned in an encyclopedia article.