Just noticed another obvious Engineer-based aspect to those two figures. Not only do they both have those same black eyes, but they're both bald (again, just like the 'melting' figure in the other concept art, indicating they could be one and the same). Also seem to have the same bridge of nose as the Engineers do.
The black eyes and lack of hair seem too coincidental to be anything other than deliberate. Definitely an Engineer influence - and probably genetically altered, as I say, considering their shorter size and Ripley/Hicks seemingly being fine around them.
Alternatively, they could be human/Engineer hybrids - which might explain the shorter height.