I've thought of something to contribute. Here's a theory of mine:
I think people focus too much on So instinct bringing a more Fe quality to Fi users, when I think it can bring more Fi to Fe-users as well. The Social instinct is a bit of a leveller; a natural bridging agent. It can cut through the some of the 'extreme' differences between Fe and Fi and creates a happy, middle ground. I think Fi or Fe simply frames the use of the Social instinct and this inadvertently gives rise to qualities that can resemble the other function.
FPs are naturally adaptive people and are interested in universal ways of being, so they connect more with the adaptable elements of So and to it's awareness of collective behaviours. It means they're more keen to shift their behaviour to accommodate another person's personality traits, interests, culture, language, or way of doing things. They still think of this as (the Fi approach of) recognising the individual and as seeing the world from their point of view, but with the Social instinct included, there is more awareness of how this person fits into to various life schemas (ie. the universal) and how this influences them as an individual - as well as increased willingness to shift to that individual's preferences.
FJs are naturally affective people and are interested in shared norms and values, so they connect more with the mediating elements of So, and with the desire for the group to be happy and successful. It means that they are more focussed on individuals (than perhaps other FJs are), and attending to and accommodating what makes them different to the group/norm. They're still more driven by (the Fe approach of) shared aspects and common ground, but with the addition of the Social instinct, there is more willingness to address the exceptions to the common yard-stick, and to perhaps shift the goal posts to include and accommodate the individual exceptions.
So in a sense the Fi and Fe can come to resemble one another when combined with the Social instinct: Fe-users become more understanding of the individual preferences and Fi users are more accommodating towards the social norms. But the interesting thing is both functions are still operating within their own bounds; it's only the application that changes, particularly in regards to the sorts of values they take on.
For example (I've probably told this story before), I met this ENFP Sx/Sp while travelling in the Middle East. She had just come from Egypt and bitched about the expectation that she should change to modest dress (ie. loose clothes covering your chest, shoulders, back, midrift and preferably most of your legs), especially when there was religious significance (she was a hard core atheist). She had flatly refused to do it because she didn't think she should have to change who she was for anyone. She was seemed so threatened by the idea of having to change even one part of herself, even temporarily. And from the way she talked, it sounded like she intentionally wore brief clothing just to piss people off - things like tight singlets and hot pants. I had the complete opposite approach. I made a real effort to dress modestly, even when it was stinking hot, because I saw it as the only right thing to do. These weren't my countries, religions (I'm also an atheist) or cultures but I felt I needed to respect the way they did things as best I could. I didn't want to upset people or make them feel uncomfortable. This is very driven by my personal values and keen interest in others ways of living; and when I did all this, I didn't feel my sense of identity was threatened in the least. It's interesting to me how differently two Fi users (and NFPs as well) could see and approach the situation - to me this has a lot to do with the Social instinct* (and perhaps maturity too!
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*Although as a disclaimer: the Social instinct can certainly lend itself to counter-culture and defiance of norms too.
