IMO Al would be acting capo of Larry's crew and then Larry would take over once he's out of jail and replace him in the rulling pannel.

Walden was sort of a bodyguard and driver for Carlo, I don't think he was number 2 or something in that crew. That's why Tony gave control over that crew to Paulie.

I assume that if the creators of the show decided to make another season they would introduce some random older guys from different crews, just like they introduced Carlo, Maffei and some other guys in the 6th season. It's hard to imagine guys in their 30s/early 40s (Benny, Walden) as captains in today's Italian-American mob.