Chickens have very small brains, and if you pull birds from the flock for a very few days and then put the bird back, they will attack like it was a stranger to the flock. So frustrating when people pull birds for what they are sure are good reasons, and then try and blend them back together.
A lot of people personificate their birds, but truly, it won't be much of a problem. A single bird does need a friend, but even then, when you introduce them, there will be squabbles. You might want more hens, though, do not add chicks to grown birds, they will kill them. Try and add equal size and slightly more numbers than the established flock. There may be a few squabbles, but should go well if you truly have enough room.
But they really do not mourn if there are other birds still remaining, and if you get rid of randy aggressive roosters, they may just cheer! (just kiddling)
Mrs K