Going by the timeline of 40k, I don't believe it's plausible that an old one created the nids to battle the c'tan and the crons. Mainly because tyranids weren't readily encountered by humanity until the time of the emperor, and even then it was more in it's recon- stage of development ( no hive fleet, mostly genestealers and related cults)The hive fleets did not become a problem until something like the early part of the 41 millenium. While it is speculated that there were in fact previous fleets throughout time, its not enough in my opinion to justify it only becoming a threat now. also, most other races would have long shared histories with the nids if they had a common creator ( much like how all the c'tan gods are in the nearly every races creation myths)
I would have to agree with the above posters that the nids look nothing more to me than super evolved locusts. They probably originated in a galaxy far from the one we know after consuming all the raw biomass in theirs. Since they come from such a far off place helps to explain their physiology being so different then what is usually encountered.