On the Gordon Freeman note: It would be plausible for him to freak out if it was years later, when/if he had settled down and 'forgot' about it.
Sure.
He's had time to put it behind him. Saved the world once and for all... Years have passed... He thought it was all over... Nobody's seen a headcrab in a decade... And then one drops out of a ceiling duct on top of him. Sudden flashbacks to Black Mesa and the resonance cascade and fighting the combine and whatnot. Guy goes catatonic for a moment, then snaps and starts swinging a shoe around like it was a crowbar. Makes sense.
But if you threw a headcrab at Freeman halfway through Episode 3 and had him go catatonic and start crying for mommy? That doesn't make sense. He just spent all of Half-Life killing the things... Then teleported out to Xen and killed some more... Then he came back from limbo and spent some more time killing headcrabs in Ravenholm... Then spent all of Episode 1 killing them... Then ran into them again in Episode 2... And now, suddenly, in Episode 3 he's going to have a breakdown at the sight of a headcrab?
You could do it... But you'd have to give the players a very good reason. You'd have to spend a chunk of time explaining that he's retired and thought they were all gone. Or you'd have to spend a chunk of time showing that all the combat and stress is taking its toll on him, and that headcrab is the last straw. And even if you were really,
really careful about it... It'd still come across as uncharacteristic and you'd have plenty of people complaining about it.