Mmmm I know I have too many birds for the coop size I have, but they free-range from sunrise to sunset and have 24 hr access to their run and rarely spend time in their coop. I actually like having more birds in their in the winter, It stays warmer and the only issues I have had with was when I had Polish, they got really picked on, I think because of their crests. Funny because my Silkies I used to have never got picked on. Now I have a second area for my "new" Silkies mainly because they are really good breeder and show quality and I wouldn't think it's so cute to end up with crossed up chicks with them. I also have Japs in with my big birds and have never had an issue with them getting picked on.
I think it depends on the birds, really. It is not like you are keeping them 4 to a cage. As long as they are able to move away from another and they are not stressed or picking at each other then they are happy with their situation. If they start picking or beating each other up, then I would separate the banties. JMHO
Wanted to add...I have 3 roosts in my coop, they are all the same, yet they all huddle together on the same one, security in numbers I guess.