I'm working on my main settlement and wanted to have all the unused companions guarding the settlement in Power Armor.

Problem is, I read that they do leave the PA after a while when you're gone, I suspect it's for simple actions such as sitting/sleeping. Kinda sad, I wanted to make some kind of BoS-like settlement.

Do Nick and Curie stay in their armors since they're synths?

I have Nick in PA and he never leaves it. Didn't even consider the fact that human NPCs would leave it to sleep but it makes sense, I guess.