If followers are simply "waiting" instead of having totally left the group, can others join while they're gone? Also, I know they will normally permanently leave after awhile, but will 20 Charisma make them wait forever where I leave them?

If a character is not in the active party (i.e., you can no longer see their portrait on the left side of your screen), then yes, you can recruit other people.
I believe that they WILL wait in one spot forever, or will at least voluntarily rejoin you after any amount of time. I'm not certain of that though.