It is very interesting, isn't it? As to enneagram typing, the very best breakdown of that sort (well, not exactly that type, but close -- rather, Meyers-Briggs) that I've ever seen of Mulder (who is a solid INFP) was done by bardsmaid, and you can read it here. I revisit it often.

That said, it is very interesting when you consider Mulder and Scully's relationship, and try and pin down who's dominant, who's submissive, and if one of them ever remains fixed in that position or not. I will point out a few obvious things, just in case: Scully, that we've seen in canon, has never initiated a kiss (though she comes close in The Truth). In FTF, she waits, absolutely still, and looks as though she's anxious, but will still allow Mulder to kiss her. Mulder is also the aggressor in Millennium, and in The Truth (when he walks to her and reaches for her).

I've always found this pattern quite interesting, considering Mulder's tendencies for self-flagellation and lack of self-confidence as to his worth as a friend, partner, and likely as something more. Then again, it is still in character, I think; Mulder is prone to leaps, both of logic and faith, and it seems that translates to other passions as well. ;)