Is En Passant Only for Pawns?

Because en passant can only occur after an opposing pawn has moved two steps forward, as a general rule pawns may only capture en passant on the 5th rank (for white) or the 4th (for black). Again, en passant is only legal the turn the two-step advance is made.

Can you en passant with any piece?

En passant is a unique privilege of pawns—other pieces cannot capture en passant. It is the only capture in chess in which the capturing piece does not replace the captured piece on its square.

Can you en passant with a bishop?

First of all, neither a rook nor a knight can make the single diagonal "en passant" move. Of course, kings, queens, and bishops can, so the question is why they do not. A pawn moving two squares represents a group of lightly armored foot soldiers moving at "double time" (speed, actually).