I wonder how much of an influence has Ridley Scott's "Prometheus" had on Bec to write series like this? But I might be wrong as theories mentioned here are out there for quite some time. That being said, alien design is something that's already seen more than enough. Was it intentional, in order to provoke some feelings in readers or the artists ran out of ideas? I'm leaning to the first explanation as the entire concept of the series is to be like Hollywood summer blockbuster.
Also, the idea of using old aircrafts to fight the aliens remind of a Croatian SF novel "Sabre". Fun read with an interesting idea, if only had it been cleaned of the nationalist crap and edited properly.