There are two important things to clarify before a party to assist your guests: 1) Requested attire and 2) food that will be offered. It appears that you may think formal dress and a formal dinner are synonymous.
“Finger foods” means either light or heavy hors d’oeuvres. My recommendation for your party would be either “heavy hors d’oeuvres” and “business attire”, or “light hors d’oeuvres” and “casual attire”. “Semi formal” and “formal” are customarily for an elegant dinner.