Use Samba4 LDAP services with other services

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.