I wonder if the hardass thing actually helps. A lot of coaches spend a lot of time trying to show how tough they are, Tortorella is doing it now, and I wonder how much it helps. You can get a bounce switching to a player coach from a tough guy, and again if you go back, but ideally you would need someone with balance. Part of the habs problem is that so many of their last coaches, Martin, Therrien, and Julien (no idea about Ducharme) were so similar. Vigneault was maybe the only coach who didn't fit the mold and he didn't get much of a chance. Carbonneau and Tremblay were tough guys, but too inexperienced to affect the discussion.
Being a hardass definitely ages badly. All the hardass coaches have a pretty definite and short term expiration date. St-Louis is already a longer than average tenured coach. If they keep him for the rebuild he'll be positively ancient in NHL coaching years. It makes sense to alternate, so the next coach should be a disciplinarian, but I don't see that as a knock on St-Louis, or a reason he needs to change.
I have no idea if St-Louis will be able to adapt. He has a pretty limited coaching background so he may well get outcoached in the playoffs. It doesn't always take much, just finding a tactic the other team can't counter. Running Moog into retirement as the goonless habs and gormless refs watched, for Ruff against Vigneault as an example.
Being nice and not picking fights or bashing players in the media isn't weak, and it doesn't mean that St-Louis can't blast or demote players out of media view, it's just a bit unusual based on the behaviour of the last few coaches. I really don't miss the mindgames, or the very obvious destruction of player value and reputation forcing the GM to dump guys. I don't miss coaches saying "I'm never going to play this guy again" while the GM looks on without comment.