If you can find them, go for it. That may be the difficulty, finding them. I have frequently added hens to my flock, and been pleased with the addition. You do need to consider that while adult birds are sturdier, they can bring in disease to your current flock if you have one. Quarantine can be difficult to set up, and if you do not do it correctly, you may as well not do it at all. Research quarantine on here.
Chickens of all ages do die. Always it is the favorite that dies or the predator gets. I find I just accept that as a fact of life and focus on the living birds, but I do realize that not everyone can do this. If a chicken death is very hard on you, perhaps chicken keeping is not for you.
Mrs K