Puzo fashioned an unusual Mafia family structure. An Irishman as a Consigliere was, of course, the most salient part of that unusual structure. However, he never cited an underboss and, apparently, he had Tom assuming some responsibilities in that area (that is not all that inconsistent with usual Mafia family structure since Consigliere's are not impotent when it comes to family operations). Puzo also structured the Corleones with only two capos to boss upwards of a thousand soldiers. Not that is not only unusual, but impractical.