Is it possible to use Samba4 LDAP services with other services (eg. authenticating Joomla users using Samba4)?
Of course, we already do it. Check for example nextcloud package. Also take a look to sssd one.
Another question, in NS 6x LDAP setup, the domain and organisational attributes where already pre-set, will it be possible to easily change these attributes for either Samba LDAP or OpenLDAP?
Yes, you can change organization details on OpenLdap and Samba.