Hell, Joe Bonanno made his too young son his underboss (and possibly his Consigliere)over the heads of much more experienced people. The novel acknowledges that an Irishman as Consigliere was Vito's only foolishness. By rights, one would want an older consigliere if only because of his accumulated experience. Tessio could have made a decent Consigliere. Afterall, "Tessio was always smarter" and "he sensed" a strength that Mike was keeping hidden. He had the instincts that's for sure.