An INFJ Six that has a lot of fear and doubt but can't identify authority issues probably isn't a Six at all. Rather, they are simply experiencing the high degree of anxiety typical of INFJ. (INFJs, in addition to reporting the greatest degree of marital dissatisfaction, also have anxiety in the greatest number of areas in their life. I'll get into the statistical correlations between the MBTI and validated Enneagram tests later in this post.)
I think part of the reason why you're finding a lot of INFJs quickly typing themselves as Sixes is because they hear about not just anxiety, but skepticism and doubt, which make Sixes seem more intellectual than some of the other enneagram types. Actually, different enneagram types may be intellectual, but in different ways. Talking about enneagram Four In Character and Neurosis, Naranjo states:"We are in the presence of an 'emotional type' just as in the case of ennea-type II, only here with a greater admixture of intellectual interests and introversion." For type Six, though, he describes an intellectualism and a commitment to reason that leaves little room for ambiguity.
Unlike ennea-type VII who uses intellect as strategy, type VI is likely to worship intellect through fanatical allegiance to reason and reason alone--as in scientism. In his need for answers in order to solve his problems, type VI is more than any other a questioner and thus a potential philosopher. Not only does he use the intellect for problem solving, he resorts to problem seeking in order to feel safe.
His sample sizes appear to be adequately large (type Two, for example, contained 200 participants). In his study, type Six was correlated with ISFJ, not INFJ. For type Four, the greatest correlation was with INFP, not INFJ. INFJ didn't directly correlate with any of the nine types, so we can assume that INFJ is not so close to any one enneagram type as to be thought as indistinguishable. In fact, if you're INFJ and type Six, then you more likely than not find at least some of the Six descriptors to be ill-suited to you, since a lot of Six descriptions emphasize the introverted sensing aspect of it.
Are you an INFJ and don't know where INFJ ends and type Six begins? Simple. INFJ's dominant function is introverted intuition but most people who test as type Six have a dominant function of introverted sensing. Don't know the difference between type Four and INFJ? Simple. Most people that test as type Four introvert their feeling whereas you, as an INFJ, extravert your feeling.