To find out what stairs you can currently challenge someone in, first go to the Main Stairs Page, then click on "Show your stairs only" link. The ones in BOLD are ones you can make a challenge in. .
What happens when some in many different stairs becomes a pawn? Since pawns are only allowed to enter 1 stair..... how is it determined which stair they stay in and how long can they remain on the leader board in stairs that they are no longer able to play in?