I have always been very interested in membercharts, including crew succession lines and breakdowns. But I have learned that it is extremely hard to follow the lines from back in the 1930s to today. Such straight lines are very rare.
Keep in mind that a boss wants to have loyal captains around him. He can basically do whatever he likes by promoting and demoting crew leaders. He can split crews up, start new ones and disband other crews however it pleases him. He also have the power to take some soldiers out of one crew and place them under a different captain in order to make that captain stronger. All this has been done, and I guess still is being done, frequently.
Sometimes soldiers are put directly under the boss. These soldiers don´t have to report to a captain. They sometimes acts as drivers for the boss or personally attends or runs the boss´s personal businesses such as loansharking or numbers operations. Greg Scarpa (when Joe Colombo was boss) and Johnny Morales (for Bonanno) comes to mind. Sometimes, if these guys have a very close relationship with the boss, they are made captains but without a crew.

In the past, the Gambino Family had the largest number of crewleaders. Since the 1930s the Gambinos was regarded as the biggest Family. Today, I believe the Genoveses have replaced them in that regard.