All just guesses:

I think that Tom's adoption occurred around age 11 and that he's the same age as Sonny, so it would be in the mid-1920s.

I think the Maranzano war takes place a few years before the "colonial war" in which Vito is wounded by the Irish gang. I think the latter occurs around 1936, and that Sonny joins the Family in between those two events.

The Epic version of the film mentions Rocco ' s secret regime, so I think Rocco had his own group and Neri took over Tessio's. I don't think there's any reason to think that Tessio's old territory was run semi - independently after Tessio was killed.