I agree with the above. Subtypes have a compulsive quality in that they protect us from our core fears, just like enneagram.
Sx core fear: I am disconnection/Obscuring identity: I am connection
Sp core fear: I am insecure, dependent/Oscuring identity: I am secure/autonomous
So core fear: I am unbelongingness/Obscuring identity: I participate and belong
I think that there is an overlap in the way people are seeing Fe types play out, assuming the So subtype would naturally go hand in hand with Fe. Fe isn't based on fears, its a cog preference that more research is linking to neural activity, and would probably serve to, by its very nature, help someone NOT be an So type.. But this just a random idea I'm throwing out, so I wouldn't argue it =)