Perhaps I just used the incorrect terminology, I'm fine with certain types of retroactive additions, such as Amanda Ripley encountering the Alien, I'm not so fine with certain other notable types of retroactive additions, such as Ellen Ripley encountering the hive in Alien while saying she doesn't recognise it later on one's contradictory and the other's non-contradictory.
As for continuity and canon overall, I simply believe with respect to the individual franchises themselves, what's "canon" to Alien only affects it, what's "canon" to Predator only affects it, yet if under AVP's canon and continuity something comes into conflict with information from one stating one thing and one stating another, I personally believe the newest information takes precedence, as such a rule set's proved reliable before.