I'm personally not stoic. I've always been called temperamental from a young age. But there's something more contained, yes. I suspect the sp/so is most contained, perhaps seconded by the so/sp. Southern Kross is notably less fiery than me. There have been a few lengthy threads where my more raw communication wasn't under much bubble wrap.
Outwardly, yes, it's different, but I totally understand the fire. I get it and feel it, but perhaps not with the same immediacy - I usually feed it through a filter when I express it. I actually struggle to break down my wall of inhibition and get to the raw stuff.
The whole fire and ice thing is fairly accurate assessment of Sx/Sp and Sp/Sx (of all types) IME. The "ice" part isn't always cold and distant - it can manifest as being relaxed, chill, focussed, steady, centred. Nor is the "fire" always aggressive and full of rage - it can manifest as being very excitable, passionate, expressive and driven (often once they get the bit between their teeth they just won't stop). Of course, it varies depending on whether they're introverts of extroverts; it can be a subjective standard relative to that person's nature (and enneatype).
To me the main thing about these two stackings is how quickly they can switch gears, and I mean that in both a positive and negative sense. I generally don't (or can't) shift my own emotional headspace that fast and I can be more easily overwhelmed when others do - my emotions move more in steady gradients (internally anyway), rather than sudden spikes and drops. I've had times where people come up to me suddenly and rip me apart for something, and I just stand there like a dodo. 10 minutes after they've stormed off, I'm really mad and want to give them a piece of my mind, but the moment has well and truly passed by then.

It's only if they hit the
secret red button of death which overrides the lag, snaps my mind into gear, and suddenly I'm totally present and ready to unleash (that or being an immediate family member

). Perhaps in a sense this can make me seem more 9-like at first glance?
This doesn't seem the case with Sx/Sps and Sp/Sxs - they seem more changeable and more prepared for sudden emotional change. OTOH, I think the So/Sps (and maybe Sp/Sos?) are better able to handle social, cultural, conversational, informational and situational shifts. I don't know, perhaps you guys will disagree.
I am so/sp and I can be very passionate and temperamental, the only difference is, that my passion is usually not turn toward a person, but more toward idea, opinions, politics, social systems, changes...I can be very passionate in debate, but cold in relationship.
Yeah, this is true of me too. Get me talking about an idea that grabs me (or irritates me) and I will come off like a Sx-dom. My intense rants about ideas have rather surprised and unnerved people in the past - either that or they laugh at me and tell me to calm down. You could say Social 4s are rather cerebral creatures and that's why ideas make them come alive, whereas the Self-pres and Sexual 4s have a more naturally instinctual bent.