staying in the shadows

Whenplaying a caseul game against someone you know is going to be in a upcoming tournament im entering and I know there a contender I like to try and play opeings I usually wouldn't simply as not to share what openings I play until the tournament. Is this ok to do or just stupid ?

Nah, it's a good idea.

If you really want, you could switch up openings to try and probe for their weaknesses. For instance, try Sicilian, French, something hypermodern, and an open game. This way, if they suck at something in particular, you might be more inclined to try it in the actual game. I've no idea if you've enough time to try all this out, but just a thought. This strategy only works if you're comfortable in all of those yourself. It's a two-edged sword. Oh, and another benefit is that knowing a dozen or so openings might scare the mediocre opponent.

Alternatively, you could just use casual play beforehand to practice your own skills, regardless of your opponent's. You could even make a "theoretically wrong" move somewhere if you want, just to test what they know.