I would go with the medium point. I would avoid the fine point on those cards. I would find a different silver pen than the the silver sharpie. Do you know how the meet and greet is set-up? I would worry about those guys crushing the card. I got this idea from a collector friend. He took some foam board about standard page size and adhered four photo corners for the card/photo to house the card/photo. Easy load and unload,. This way the signer can hold the board and sign with the free hand. No need to find a hard surface or handle the card. You could make a larger board to house more cards.
https://www.autographu.com/the-best-...ng-autographs/
Since it sounds like you are getting multiples of the same guest. Create a page for each guest and just knockout each guest in one swoop.