He can only plead the Fifth if testifying will incriminate him
And Congress has the power to cite him for contempt like a judge presiding over a grand jury hearing.
Furthermore, Pretreaus is likely still listed on active duty , and any case, it is possible for any flag officer can be subjected to Uniform Code of Military Justice.
Not only am I not a Constitutional scholar,I'm not even a lawyer.However,I think that a person can take the 5th when he fears prosecution even if that fear is unfounded in law.However,if offered immunity from prosecution (which I suspect Congress has the authority to do) he *could* be compelled to testify.Below is the relevant portion of the Amendment.
..nor shall be compelled in any criminal case to be a witness against himself..
Fairly vague IMO...which,if I'm correct,would give a potential accused wide latitude in deciding when he/she could be charged with a crime.