The problem with one-shot Hellveticans is that they (mostly) don't have to think about their ammo restriction. So if four Hells can dump their whole mag into the psychonaut, your boss will be red mist in a few rounds.
I don't have the rulebook on hand right now, but you should check if the armour makes the Hells immune to the pheromones (respiration system for working in hazardous environments etc.).
Maybe make them a bit beat up equipment-wise and starved for ammo from a previous assignment.
Yeah, that's my biggest fear. The Pheromancers aren't particularly strong in a fight, their main strength is to enslave people via pheromones and Hellveticans are mainly balanced via their ammo restrictions in prolonged campaigns.
Since it's only going to be a one-shot, I wanted to bend the rules concerning the equippement a little (also, since the game doesn't offer that much stuff, tbh.)
It's going to be 3 players and I planned to start them off as corporals (without min-maxing like crazy, it's unlikely to get above Rank 2 on the start) with different skillsets, so they would fill the roles of Medic, Scout and Grenadier, where the "Medic" and "Scout" have a lighter armor and possibly not their Trailblazers. If I went by the rules, everyone would carry a trailblazer and harness and I think that would be sort of boring for such an occassion. The Medic obviously has access to a Field Kit and a lighter version of the Harness (possibly with a built-in Gas mask / "Sealed" Quality, similar to the Spitalian suits), the scout has a Pathfinder (Basically a GPS thingy) and a Recon Harness with a Camo Suit over it and binoculars. I thought about giving the Medic a shotgun and the Scout that Stubbed Trailblazer Variant (half the range, same damage as the regular Trailblazer).
As far as I can see, the Hellvetican Harness does come with a face mask, but not with a gas mask or filter system, so adding that to the Medic's Harness might make the character feel a little more unique.
In terms of plot, the general idea is that the characters are from a fairly insignificant Outpost somewhat close to the Frankan border, which received an emergency call from a scouting group of Spitalians and Hellvetics (originally from Toulon) that were ambushed and had to hole up somewhere in the Rhone Swamps. Said Outpost has had a stretch of bad luck recently, making them starved for resources and just to top it all off: One of the (NPC) Corporals recently lost his Trailblazer, meaning that his Squad (the PCs) have to make up for that failure. This would not only explain why a low ranking group are send out (they are expandable when the Outpost has nothing better to spare) but also why they are short on ammo themselves. Travel hijinks ensue.
When the PCs finally get to the origin of the emergency call, all they find is one dead Hellvetican Radio Beam Unit soldier and a half-dead Spitalian. While the Medic patches up the Spitalian, the bad guys show up. I want to beef up that encounter by putting one Hellvetican Grenadier with his Trailblazer and Harness and a Spitalian with a Splayer (both of them a tad injured from when they were captured) under pheromone influence as well as a shock troop of 3 or 4 Leperos alongside the Pheromancer Expatriate against the players.
The Leperos will be dispatched fairly quickly, but while that happens, it should get the Spitalian close enough to be a threat while the Grenadier takes potshots. The Pheromancer will do some Pheromancing, as one should expect.
I want to give my players the option of defeating the Spitalian and Hellvetican without killing them, possibly via injection of narcotics or an improvised stun grenade or something, if possible. Though that depends on what the PCs will decide to do, obviously.