In order to set a page 'read only for un-authentificated users' :
edit the configuration file wikiconfig.py with
acl_rights_before = u"AnAdminAccount:read,write,delete,revert,admin"
acl_rights_default = u"AnAdminAccount:read,write,delete,revert,admin"
Add on top of the wiki page that you would like to open for the public :
#acl MyUserName:read,write,admin,revert,delete All:read