Studio 54 is a Roundabout Theatre, which means they're probably not going to be sending out CAST-SIGNED Playbills. I've received successes from plenty of actors there, Matthew Risch with the Pal Joey revival that just ended March 1. Use the address they have on here, as Studio 54 has a different Stage Door entrance, and seems like they only get mail through the front office address (or whichever one is on here).
Previews just means they're performing in front of a full, live, paying audience for the first times, and tightening the show for the opening, should anything be changed. I don't think the previews for this one is too long. Previews (after about a week) are also for the Reviewers

. There no different usually for a play (maybe length, for instance), but for a musical it could change drastically during previews (anything from writing out a character to changing song orders...), so you're safe for "...Godot."
Good luck!