Yes it is possible to have some users in tiki and some in ldap
The option is in admin->login->ldap:
Use Tiki authentication for users created in tiki
And better to keep
Use Tiki authentication for Admin login
Perhaps you will not be able to create a user with the name anonymous - it is a reserved name.. but you will need to know the password of this user to log-in
YOu also possibility to post in forum really as anonymous - no login from LDAP and tiki . Posters will leave a name - whatever. If necessary, use moderation feature to have an admin filter anonymous posts.