But that generates my next question: these PCs know enough about what "dungeoneering" is to understand what a proper "combined arms" force looks like; but are completely ignorant of trolls' vulnerability to fire.The party composition piece, though, is very easily explainable in the fiction: in-character someone looks at the group assembled in the tavern and says "OK, we've got two sneaky-types, a wizard, and me: a bard. Guys, I think we'd better recruit ourselves a healer and a front-line tank or two 'cause if we don't you just know those'll be exactly what we find we really need once crap gets real out there."
They know that dungeons have traps that the "sneaky-types" might spot and disarm; they know that dungeons have monsters who will hurt them, thus generating a need for healing; but they don't know which of those monsters is vulnerable to what sort of damage the wizard might do.
That's a very arbitrary set of stipulations about "common knowledge".