They have 10hp. Corrosion resistant or otherwise makes no difference, as a glancing shot from any interceptor will kill them. (although I agree they should be able to fly through the clouds)
So make them higher HP, they can still be less than the other ships. Yes, they're less durable than a real fighter, but so what? That's what SLFs ARE. Putting the non-guardian SLFs against NPCs in a Combat Zone would also have that probem. And a problem that occurs everywhere is not ACTUALLY a problem in that one place.
In high risk situations, the SLF is nothing more, and could never be any more, than a drone fighter (see Fighter Drones in the X seris of games), you put them out to take some of the heat off your ship. If they were 90% resistant to corrosion (I do not believe they should be 100% immune), they would be able to be left to AI use as throwaway fodder or by you so as to remove the pests (like, again in X2, the M5 fighter docked at your M6 corvette) and help the AI pilot survive. Or, if you winged up with a human, as the best of both worlds. IMO this is how an AX strike should work: Two people, one flying the SLF and one flying the main ship, the SLF can afford to fly rings around the furball because it is largely immune to environmental damage, while the main ship deals most of the damage.
So take out three low level NPCs for your AX fight and expend them in your guardian SLFs. They're there to take some of the heat off you, nothing more.
On a similar subject, I would have liked to have seen the AX weapons be brutal againt thargoids and the guardian weapons be equal to standard weapons for human targets and STILL at least NEARLY as useful against thargoids. If you go out thargoid hunting in that setup, you pick as many AX weapons as you can, and if you have spares, you fit guardian weapons, which then become useful in self defense outside the bug zone. If you HAVE guardian weapons, you may decide to ALWAYS run with them, because they're "as good as" your normal weapons, but still very viable if you find an alien combat zone.
But with AX weapons buffed so they're BRUTAL against aliens, you will choose to fit AX weapons to as many hardpoints as you can, if you are deliberately going for thargoid hunting, you only fit normal weapons if you have spare hardpoints OR you worry about getting attacked by human ships on the way there or back.
It could be headcannoned that the AX weapons got a free buff from the knowledge gained in guardian tech, and retrofitted for free because the thargoids are now a threat to the systems, not just PF pilots and NPC ships.
If necessary, the buff to guardian weapons could be done via engineering being made available that buffed their output so that they are as good as equivalently engineered normal weapons against humanity's ships (so they can be fitted as as normal standard practice) AND they're at least nearly as good at thargoids, if less so than AX weapons, keeping AX weapons useful.