I agree with you on the goo.
This video explains the goo's properties pretty well, it also fits with the original Alien: Engineers script ideas about the goo (back there a swarm of black scarabs) infusing Fifield with Xenomorph DNA.
Thing is, the Deacon/Neomorph seem to be the ultimate creature the goo will produce by itself, as shown by the mural as well which is coherent within the context and timeline of Prometheus. Those creatures might either be an incidental result of the mutations of the xenovirus or an intended 'cleanup' mechanism designed by the Engineers.
Either way, the Xenomorph David designed wouldn't have happened by chance, nor would the virus have produced them down the line. He got two different species, the infected endoparasitary arthropod and the Neomorph and manipulated them so the former would implant a bigger, more inteligent and resistant version of the Neomorph into a host.
So, even though we are right to consider them to be part of the same "family" it is undeniably true that David 'created' a more refined and evolved version which would have never, ever, existed if a super-intelligent A.I. hadn't been performing crossbreeding experiments for a decade. That's a fact.