I think it would be beneficial to start mages with a basic spell scroll that matches their elemental skills. Likely, the element that has the most points placed in it, or random if points are equally distributed.
I understand that they are prisoners and come to the dungeon naked. Though, let's just assume the mage was smart enough to hide a spell scroll in a place they likely didn't check.
![Wink ;)](./images/smilies/icon_e_wink.gif)