Hello,
This is an excellent question. At this time we only have the two sizes of holders, the standard sized and the thick holders. If the cards do not fit in one of these two holders appropriately, we will not slab them and risk damaging them. Although it may seem that the cards are the same size, some manufacturing defects can cause the cards to fit more snuggly or in the other cards case, loosely in the holder than others.
With the card that is a little loose, we generally will put a sleeve on the cards that we feel will benefit from this method to prevent movement but if you feel the movement is excessive, you can send it back for review and we can see about adding a sleeve to the card.