If one monster is affected (positively or negatively) by fire/flame weapons, are all of them?

A monster's immunity (or weakness) is given to all others in the fight. (This does not include straight combat bonuses or penalties; if one monster gets a -2, that doesn't mean EVERY monster has a cumulative -2 – it's -2 for the monsters taken as a group. And if a monster won't fight, that doesn't mean EVERY monster won't fight, just that one.) If it helps, think of the combat as a combined encounter, and the various immunities or weaknesses as conditions affecting the encounter.