Changelings

[Firepower]

I don't agree.
Changlings are probably one of the most balanced antagonists. They're not meant to be a 1 vs 1 type creature, they're meant to be taken down with at least 2 other people. It was unfortunate that your other officer got spat and absorbed, but moving into deal with the ling without any back-up at all was a bad move. You could've gotten any passerby, a clown, a staff assistant, throw him a gun and tell him there's a ling there, and back you up if shit went downhill.
I normally alert an AI via PDA, once the AI isn't some sort of sadistic lunatic that will demand that I kill the ling and will actually help me capture it by shocking/locking doors.

THAT SAID

The situation you highlighted is very typical. No security back-up means that the opportunity of having back-up that isn't a clown is minimal. Having 2 officers (Including the HoS, excluding the detective as he could potentially be an baddie) is normally average, so one dies, the other officer is quite fucked against the ling.

Officer helmets (including the ones in the lockers, and the hos beret/hat) could perhaps be buffed against a ling spit. Not block it, but reduce the stun-time to a point where the ling would probably want to sting them afterwards and drag them out a window into space.
It's less "can't spit this officer because he's wearing a helmet"
and more "I'll spit this officer when i'm more prepared"

Even 2v1 the changeling has a pretty good chance in fights. Considering he can acid spit the first one and neurotoxin the other if he gets too close it's pretty ridiculous. You need an entire angry mob to take out a changeling basically. The acid spit's cooldown is far too low, if I catch the changeling with his pants down after he just absorbed a person I SHOULD have the upper hand on him. The changeling was super careless by just running into the security office at the start and spitting the only officer.

The changeling has some of the most insane abilities ever, they can transform to hide if they get found out, go to lesser form to escape handcuffs, spit homing acid at people even while stunned, neurotoxin people, drug people, kill people extremely fast by absorbing them, heal himself back to full health without any medical supplies, space walk. if it dies it can take over another body which effectively gives it a second chance, and lets not forget if it actually does get into a fight it can't win (which is rare these days) it can always go into horror form and pretty much fight a small army provided they aren't heavily armed with guns.