Assalaamu alaykum,
we can write 'Bismillah..' on the marraige card, inshaAllaah. what I am saying is - It will be better from the islaamic perspective if we don't keep ayat or hadeeth in the card. And some of the reasons I did mentioned for that. There is nothing compulsory to put in ayat or hadeeth in the card, and especially I do concern about the sanctity of them. InshaAllaah, hope it makes a sense and Please do refer the link. wassalaamu alaykum.