1) Kobolds are only able to avoid traps while sneaking. A visible kobold is treated like any regular enemy. It's also not guaranteed to avoid traps, you'll sometimes have them trigger a weapon trap without being spotted. Either way, a guard dog or Animal Powered Watchtower (see my signature) with cage traps further away from it, means that the thief gets detected, loses immunity, and runs outwards and into a cage trap he just snuck over.
2) When eggs were introduced, all wild animals would claim nest boxes. This resulted in hilarity as crundles and wild turkey would rush through your heavily fortified base, causing all your civilians to panic because there's wild animals running through the halls, and then kill all your livestock and claim your nests. It also resulted in less hilarious (to the victim) moments where cave crocs would run through your fort, slaughtering all your legendary workers as they passed through the dining hall on the way to the surface where you had your nests tucked away. Kobolds, under the same type of imperative, would immediately seek out an empty nest box as soon as they entered the map, thus making the perfect kobold trap nothing more than a nest behind a cage.
This has been fixed, and currently egg-laying wild animals will not claim a nest at all. This is a problem, as it means egg-laying animals will never breed until tamed. The Mermaid Farm from days of yore is impossible, nor can you breed wild crundles to train your militia against. I consider this a stop-gap measure to prevent cave croc invasions, but clearly wild animals have reproduced just fine without carefully lain nest boxes.
Either way, you cannot have them as domestic animals AND as a civ, the [PET] token causes strangeness when used with civ creatures. They will breed, however, as I have experienced elves in captivity breeding just fine, though I can't recall if these were [PET] elves or vanilla. Either way, it's possible without breaking the game, to breed captured kobolds.