There are many reasons behind this. One reason that is that I've looked after Ex Battery hens in an otherwise mixed group.
What some people want chickens to behave like because they added chickens like getting a bag of sweets from a pick and mix store.
What chickens are really like.
Chickens are tribal creatures. They are not flock creatures. Chicken will identify with their own breed, own family, or whatever features another breed has that is closest to their features.
There are certain mixes that should be avoided; Silkies, Polish and heavily crested breeds mixed with standard full sized breeds. Doubtless some people will say this is nonsense. There are a couple of long running threads here where this mix has been forced upon both types and if you were to read the threads you would see it's been a disaster from the outset. Unfortunately people have an uncanny ability to ignore the probemls their lack of understanding has brought about.
Common view here on BYC is that a rooster needs ten hens to avoid overmating etc. The truth is a rooster will have two or three favourite hens and his attentions in both mating and care will be directed at his favourites. In a confined setting this isn't much of a problem; free ranging it can prove very costly because a rooster cannot provide the duties expected of him for more than three or four hens.
This is in part because his ancestors had small tribes comprising one or two females and their offspring for a period of time. No jungle fowl rooster has a harem.
Unfortunately no evidence or experience outweighs what people want whether it's good for the chickens or not.
You have possibly the worst combintaion and you are trying to introduce a new rooster (you don't mention the breed) into a unstable group. You have it seems already found out that the red sex links are not going to mix well with your Silkie and Sussex.
