It is nice to see that this question is alive and kicking 10 years on. Having read it again, I thought about how I would answer it now. Here's another answer with some insight from other games I read, run and played in the meantime.
Think about the outcomes of the situation at hand. There's this dangerous person disguised as a stoner. What happens if they can't see through the threat? What happens if they do?
I'd say, don't roll until the moment of reckoning comes. Maybe the fake stoner will spring on them when they turn their back. Then the roll doesn't come until that moment.
Then of course, it is an ambush/surprise attack of a sort. I'd frame it as an attack action, using deceit versus empathy, hitting the physical stress track. If the fake stoner succeeds, then somebody gets hurt right away.
There are two caveats: One: anybody in the party could possibly see through the disguise. Two: possibly only one PC gets hurt (unless the attack involves something that could hurt everyone)
My solution to both of those is just having the highest-empathy PC in the party defend. As per the Fate toolkit, other PC's with the same level of empathy may add +1 to the roll as some sort of passive assistance. This happens independent of who gets hurt on a successful ambush. The defender defends the whole party.
As a bonus, this is more streamlined than having everyone roll for the same thing. It also avoids skewing the odds due to multiple rolls. And it launches the action right away.
An even more streamlined approach would be having this single roll work both ways, like a counterattack. If the PC's see through the disguise, then they get the drop on the fake stoner, who gets hit on the physical stress track right away. Then the rest could be your usual conflict.