Salusa Sacundus is never seen in the book, Paul I believe unravels maybe in a vision why the Sardukar are such terrific and terrifying soldiers.
The Emperors turned Salusa into a secret prison planet, with no walls or rules, except survival.
The toughest survivors are inducted into the Sardukar.
Only Arrakis is a more brutal planet to live on, hence the fremen are slightly better fighters.
And once Paul and Gurrney train the fremen in proper military tactics, they become a unstoppable military force. That even the feared Sardukar can't handle.
Which is a long rambling way to explain why Irulan has to buy a Wedding Dress!