Newbold is picking up steam after their hiatus last year(ish), the latest alien type greens on the show are Quist Micronauts. Those looking for somewhat smaller humanoids should like them:
http://newboldworld.com/S08Arr/S08Arr23.html
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Also, a post on TMP alerted me to the Laserburn Flesharons. Apparently they are actually quite nicely sculpted and very large for their nominal scale of 15mm, so they would work as small nonhumanoids (PLA mutants, anyone? :wink:) when mixed with 28mm scale minis. That, or as size 3 minis in their stated scale. Available (but poorly represented in the pix, apparently) here:
http://www.15mm.co.uk/Flesharon_Aliens.htm
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Furthermore, producers of somewhat alternative fantasy stuff Dragonblood Miniatures has released a "Defiance compatible" mini in the form of Clal-Chk Firesting Sniper. This large (40mm) biped insectoid armed with a long rifle wouldn't look too out of place on a scifi tabletop (especially one of "future gothic" persuasions) and is available singly or in packs of 5 (they are all the same scuplt AFAIK, but looks easy to convert at least for the poses):
http://www.dragonbloodminis.com/Clal-Chk_Fire-sting_Sniper.htm
The main site seems to indicate further Clal-Chk models may be on their way.