Here's my take on the worrying of A:C shitting on the canon in some way.
As much as a alien fan as I am, I am more so a fan of good movies. As long as this movie is well directed, well shot, visually stunning, well acted, good dialogue, suspenseful, tense and doesn't feel rushed in the third act (my main problem with Prometheus) then I can look past not following canon or whatever. I don't even have much of a problem with David creating the Xeno, as long as it is executed well. If the film itself it quality, then I will be a happy man.