2nd try with an idea !

Ok, little idea here. There's more than one David on the Prometheus but they aint activated yet ( Spares ), When Active-David finds the Goo and gets experimenty with it he tries it out on the others first and thats why you have a number of blue guys who all look the same, they are all Goo evolved Davids. The guys with Elephant like proboscis are the species which built the ships and all the cool tech but the Davids are, well, Davids who can now use it.