If your read the stories on the Foundation --and before-- you will find a constant: all those stories relate to very real and attested practices and customs known to many a primitive society. That is the kernel of truth.
The tale of Hercules and Cacus --already a legend in Romulus' time-- is the story of a cattle raid. The Irish have left the beautiful story of Cuchullain's cattle raid. A bit embellished, I reckon..
The custom of the asylum --a sacred place where fugitives could be safe-- is also very well attested and it is indeed a very good way to attract a population.
The absorption of the population of Alba Longa is also an anthropologic constant. Tribes get absorbed by other tribes. It happened in Germany during the 2nd 3rd centuries AD and propbably later. The "tribe" of the Alamanni, for instance, was just the "tribe" of "all men", meaning they came from everywhere.
The rape of the Sabines is a custom known as wife snatching, still followed --in a ritualistic way-- in some african populations.
As for the she-wolf... Well, every Roman knew that in latin, "lupa" means not only she-wolf...
A hint: in french, a brothel is called a "lupanar"...