HI Fabian,
IMC as far as I can tell can only do AD (Radius) and LDAP authentication.
Tacacs is usually an authentication method that is used to auth devices in as far as I know. For application and other devices, a directory (local db, radius, sql, open directory, etc) is required.
You can authenticate users (IMC operators) via clearpass with AD, LDAP (or use local db in clearpass) ~ let me know if you want more info on this and I can try to assist when I have a chance ?