I thought I was the only one! The 6-limbed creatures/4-limbed humanoid thing bothered me for a few hours after the movie, too.
Gee, if we weren’t assaulted on every front with the “news” that liberals are so much smarter than conservatives, one might wonder if James Cameron didn’t know what the heck evolution was or why he believed in it...
I noticed that six legged thing too. Since my paradigm considers creation as opposed to evolution, I just ran with it. The six legs are the low hanging fruit. There were many other inconsistencies as well. I get used to ignoring that when reading fiction as long as it is not too relevant.