Ahh I agree with this for me, yeah. I'd say that some energy for so/sp and sx/sp goes into their personal well-being and comfort, but from an so-first perspective it makes us see sx/sp as just averse to this kind of "group" interaction and from an sx-last perspective it makes us see so/sp as "stingy." After talking to @The Great One (who I believe is and types as so-last) we talked about our concerns and we realized I don't really consider physical danger when making decisions and he doesn't really consider emotional danger as much. On the flip side, he takes into account physical/financial concerns when making decisions and I take into account my emotional well-being. I think if we were to attribute this openness to so/sx it could possibly be a form of minimizing our chances of emotional harm? Sort of like a dog showing someone its belly - a reveal of trust to secure our bond with people and to remind them to not hurt us.But in all seriousness -- say you have a set amount of interpersonal energy. The way I see it is:
so/sx: Use most towards "the group" on a shallow level, the rest on deeper "merging" interactions with people close to you, with little to no energy left over
so/sp: Use some towards "the group", stingy enough with the rest to be resistant towards using much for deep "merging" interaction
sx/sp: Use most towards "merging" interactions, skeptical about wasting the remaining energy on shallower "group" interaction
So your sort of first-come, first serve openness towards just about anyone who interacts with you is what I see as so/sx. I don't attribute that to Fe because I'm like that (I think) and I'm so/sx as well.