You are basically correct but it’s not a problem for this plan.
Feather Fall can only target creatures that are already falling. It’s clear about that.
But they can all go to the ledge and say “Now we all jump off and I cast Feather Fall”, right? If you weren’t running on combat turns, that would be sufficient. You’re not going to go around the table asking what order they jump off.
In terms of the turn-based mechanics, the way to represent this is for everyone except the sorcerer to go to the edge of the cliff, and ready an action to jump off when the sorcerer jumps. Then the sorcerer jumps, they all use their reactions to jump, and the sorcerer casts Feather Fall on the whole group. (In general, any time you have a plan that involves coordinating the timing of your actions, you should think “readied actions”.)
It’s not necessarily obvious that this is the way to use the mechanics to get what they want. In the play style I’m accustomed to, the players’ job is to decide what their characters will try to do, and if they need help representing that in the mechanics, the DM should provide that help. (Note that you are still using the mechanics–you’re not fudging and saying “OK, sure, you can all take your turn at the same time”. For example, they could get attacked between readying their actions and actually jumping. This is a reasonable consequence of trying this plan.) Since you are asking here, I assume you’re following a play style similar to that.