Re Alien Earth, the series creator introduced the 4 other species as if it just had the xenos it would be a 2 hour film, rather than an 8 hour series. In other words the new species broaden out the world of the show rather than just being monster of the week fare.
I'll just completely contradict myself now by saying the most successful aspects of the show have been to do with the crash of the Maginot, and how this recreated the first movie. So does that mean I just want a rehash of alien/aliens? No, cos I think the new species have been really positive, from those disgusting blood ticks, metal sucking alien flies that can wreck synths, eyeball monster (MVP imho), moving plant pod as well as the xenos.
I think placing this series just 2 years before the original film makes things needlesslly complicated. In the films we've heard of synths, but certainly not of hybrids, equally no mentions of Prodigy Corporation or Boy. Therefore at the eventual end of the series does it just reset to what we know?
Gotta ask though, how can Weyland collect 5 different species using a specialist science crew possibly up to 65 years before the Nostromo gets directed to land on LV426? They make home (nearly) and get undone by sabotage and stupidity.
If FTL drives are due to be invented any day now in the series, What's to stop the company sending a state of the art ship and crew to round up some more critters? So many questions!