Hmm...do Kobolds ever send sieges, or just thieves?
On the one hand, the "friendly" kobolds encountering a single kobold thief from the same civ will result in a loyalty cascade, so you'll want to isolate them from kobold thieves if you want to keep them alive.
On the other hand, in my experience with bugged "friendly" goblins on reclaim, they'll stay friendly exactly until you get a siege. When you get a siege by their civ, they'll become "hostile". I lost a few dwarves that way to a goblin that I'd let live in one of my bedrooms, since it was friendly and not getting in anyone's way.
If you move in with the kobolds, live side-by-side in peace and harmony, and get a kobold siege, you'll be living side-by-side in pieces pretty quick if you haven't armed and armored your civilians in case of betrayal.
A "hostile" will attack an "invader", so they still lead to a loyalty cascade that results in only one remaining "invader" or one remaining "hostile" when two groups meet.
One caveat: since thieves and ambushes don't trigger the change from "friendly" to "hostile", any goblin/kobold that survives a loyalty cascade caused by a thief/ambusher will not change from "friendly" to "hostile" when a siege happens, since they don't care at all whether their former civ is attacking anybody. They'll still trigger loyalty cascades in invaders they meet, but won't turn on you.