I never really liked the idea of the "dog alien". A lot of "bug" lay eggs in other bugs but they never take anything genetically from their host. It would make their evolution too unstable.
The fact that the Aliens didn't have the transparent layer on their head was due to production costs and time being limited. A few from the crew also thought it look "cooler". I could recheck that fact on my Cinefex.
I prefer the transparent dome look myself, but can't stand the human upright stance from the first movie. There's a director's cut coming out, maybe Ridley Scott did some special edition type stuff. I read he also hated the same thing about his own interpretation.
I thought the Aliens swimming on the fourth was pretty cool, but thought The final alien hybrid was lacking in design.
Just like Drifand, I completely disclude the third movie from the rest.