Enneagram bears a similarity to the five temperament system I always talk about.
People see the loyalty to authority and right away think SJ, but the fifth temperament, Supine is loyal as well. The Supine in the 16 types would fit ISF/INP or NF. The loyalty would be in the area called "Control", rather than Inclusion (social skills or Interaction Style), and Supine has more of a dependency need than generally described for NF, which seems to be somewhere between Supine and Phlegmatic. (Dependency does not seem to be directly associated with any type group, though I have seen NF's say they have trouble making decisions).
Both Supine and Melancholy have low expressed behavior, which in Control would correspond to what Keirsey called "Cooperative". (Berens, "Affiliative"). This is where the loyalty comes from. So the NF and SJ both have it in common, but because the NF is more moderate in behavior, it never got quite the same reputation for "loyalty" the SJ got. But it is not really incompatible with it. For one thing, the SJ's loyalty is perhaps more visible because of the fact that it is concrete focused, where the NF is abstract.
So if the person is a 6, he could be either INP type, but the loyalty points to [IN]F. (NT's being pragmatic are more prone to challenge authority).
Now, you're saying INFJ, which would fit Melancholy-Supine, and thus still be compatible with a 6. (I hear they're often 1's or 4's, but I think 6 comes up a lot as well).