Sorcha,
You mention that "... RC Latin/Hispanic Americans score almost off the MMPI scale for schizophrenia... ."  Unless the test has been re-standardized using that populaton, that fact is rather meaningless.  The validity of the MMPI (or any such test ) as a diagnostic instrument has to be determined empirically for whatever population is being tested.

I'm not up to date on the more recent validity studies of the MMPI, but at the time the test was originally designed, there were few Hispanics included in the standardization samples.  Answers that may distinguish schizophrenics from non-psychotic folks in a population of protestants, Jews, atheists, etc., may not distinguish between them in a population of Roman Catholic Hispanic Americans.