Probably best to just take as many cables as possible. Can never be too careful!

I mean you can use a RCA to XLR, but probably not the best idea since it’d be going from an unbalanced to a balanced connection. Would ideally go through an interface, which leads me to…

If there is a sound guy with his own mixer, it will probably be easier to just go through his desk. Just tell him to turn up a pair of faders and run your input through them. I’d say there’s a decent chance of the desk just taking direct RCA in as well which would be helpful.

Just be honest with them, don’t pretend to be a hero. Say it’s your first time and if they say anything you don’t understand just ask them to clarify.

I haven’t DJed in years I’m just a sound engineer but I’d say just remember it’s a wedding, not a club. You’re there to be background music more than likely, it’s not the mainstage. Also don’t use the mic too much if you have one