I certainly think that when you capture an intelligent creature (are kobolds intelligent? never checked, but same goes for them) it should become a 'Prisoner'. They could require food and drink to live, have there equipment taken off them and have their thoughts etc viewable.
Oh, and when I say they need to eat and drink, I don't mean that it should be compulsory for you to do this, just that they might die if you don't... maybe a overall toggle "feed prisoners Yes/No" in orders and then specific orders on cages containing prisoners that would over-write the main orders (For example, Feed Prisoners: No" might be the setting for your fort, but on the cage with the goblin king you could activate to feed him, keeping him alive while the others are left to starve.
Also, removing equipment from prisoners should be possible without taking them from the cage (and maybe make it so that you can assign equipment to prisoners in the cage as well? If the is a limit of one prisoner per cage then you could just give a job at the cage to "remove item: etc" or "add item: etc"... This'd certainly make setting up arena fights interesting (a "released prisoner" would still be a prisoner (so you could see thoughts), just Hostile as well)