The grey child was in fact changed before the games release because it resembled more so a real child and to kill with the iron pipe proved very voilent.. interesting the things they changed.. off topic that so back on topic.
I went through all pics of alien convenant and the burned corpses i seen where in facted very big and muscular, indicating they actually are true engineers. Also the pic with the engineers close to the temple, the first image released, shows the genitalias to.. it is all very specific.. if you examine it rigt younwill see this.. small to for engineers lol.. but yeah definitely engineers.