Even if I use the alien ants AS giant ants, you got to describe them more like Starship Trooper bugs/Tremors movie or embrace the 1950s cheese.
For Atlantians, I'll go with describing their tech as biotech. Things are symbiotic and you don't know they are wearing environment suits until you kill them and the suit sloughs off. that will make them seem more alien and players less liable to take their stuff.
Ok, I get the whole argument against adding stress against one set of characteristics when the test is against a different set. But I can see the flip side of the coin as well, and have seen it in real life during quite a few firearms training classes. Disclaimer: I am a certified pistol instructor, so I will use the lone gunwoman example to illustrate what happens to some of my students.
Let's assume just target shooting for a minute (no zombie):
First shot, student has no experience, listens to my every command, smooth trigger pull, easy breathing, etc.... BAM = 10 ring (usually a bullseye). Ok, so student never shot a gun before, took their time, concentrated and has now demonstrated that they can shoot perfectly fine.
Second shot, less coaching, let student perform most of the actions while observing... BAM = 9 ring (first shot is usually the best, next shot they usually relax and shortcut something)
Shots 3 - 5, let student shoot on own while I start student in next shooting stall... BAM BAM BAM = shots start drifting around
Come back in a few shots to find the student cursing and saying they can't shoot. They don't know how...
What does this demonstrate? The shooter obviously has the skills, they demonstrated it with the first shot. What happens is they let the next few shots GET TO THEIR HEAD. Their fingers didn't break, their hands didn't get bitten, they started to shortcut the steps and they let MENTAL doubt creep into their shooting and prevent them from performing PHYSICALLY.
And the more pissed they get, the worse they shoot.
I send them for a break and a soda, they come back and we do it again and they do fine.
OK, so let's extrapolate that to the gunwoman scenario from earlier. Woman shoots at zombie a far ways away and missed. Dang it, she missed and now it's closer. There is still not much of a threat, but she just wasted a bullet and it's weighing on her that she only has a dozen left. She lines up the next shot and the bullet grazes the headbox, but doesn't down the zombie. Gunwoman is cursing like a sailor (quietly of course, so as not to attract even more zombies than the two shots already are). Now the zombie is getting in threat range, she's wasted two bullets, she's made a bunch of noise. On the third shot, she may well be able to put the muzzle of the gun against the temple of the slow moving zombie and make gray matter stew... but the stress of the ordeal may have drained her mentally, rather than tax her physically.
Now, that might be playing too much to reality and not into the RAW, but it should demonstrate how it is POSSIBLE that a physical action can drain you mentally or emotionally (Stress you as it were).
However you play it with your group though, be sure they you all agree. After all, the rules are an abstraction and if you want to keep tests and stress in the same category, I fully support it. It's probably how I'll play the first few times. Yet if your games go static and you want to explore the cross stress mechanic to add spice, talk it over with your players.
Either way, just make sure everyone is having fun.