Or you could always do it before... Jack's appearance? That would mean there would also be more sane people (hopefully) at the place and that way you could add more plot twists if you wanted to. Plus Adam would be more than easily obtained then.
You could always have "more" of those audio diaries then too - as they wouldn't have been picked up by curious Jack.
The storyline could be that the rumours of the war starting so certain people run away to Rapture in seek of a safe place. Of course the brochure forgot to add that most people are turning insane / mutating because of plasmids and slaughtering one another.
Could make it even bloodier than Bioshock, seeing as there will be more people alive, and trying to survive. The longer occupants stay there, the more violent, insane it gets.
But whatever, that's my two cents.