I would like to organize an engine tournament on Arena and restrict it only to one opening. How do I do that?
When you set up a tournament in Arena, there should be a tab "Options" and within that a selection thing for Starting position. There you can select to load the starting position from a pgn file.
If you do not have a pgn yet of the opening you want them to play you can e.g. play the desired opening line in Arena and then File -> Save as pgn. If you want the tournament to test multiple variations you can create a fresh board, play the next line and then save it to the same pgn.