The flock master, or your top roosters stay with the flock all the time for breeding, pecking order and limited flock protection. They do tend to pay attention for possible predators, but not till they are over a year old. A good flock master is a joy to have, a rotten one can be a dangerous nightmare, especially to smaller women and children.
 
Generally, the rule of thumb is, 1 rooster/ 12 hens..... kind of sort of.  Too many roosters, or the wrong roosters, and you have a cock fighting, over bred hens, and a lot of tension in the flock. 
 
A good rooster - manages the flock dynamics, finds food for the girls, keeps an eye out for predators, does the wing dance, keeps younger cockerels in line, and does not attack humans, is fertile, and adds good characteristics to the flock. Most of us, try and get at least some of these traits, few of us get all of them in a rooster. Personally, I think you get better roosters raised in the flock under a broody hen. 
 
Roosters that are juveniles can be raised right in the flock with their pullet flock-mates, up to a point. If you have a flock master, as in an older established rooster, they can stay very close to 4-5 months. At that time, puberty begins, and depending on the roosters involved, fighting can start. Some fight once, and get the pecking and breeding order established and are good to go for long periods of time, others it can be a daily fight, or even a till death fight. This is the time that you need to separate these roosters away from the laying flock. These are the ones that will be meat. Merely separating them with a fence will NOT work, as they often will fight bloody through the fence. These are the roosters that need to be out of sight and sound of the hens, in a  rooster pad. They will calm down, and put on weight. If they are not free ranged, they will be more tender.
 
If you incubate chicks, then you have to do things differently. The laying flock will kill these chicks if introduced too young. Many people firmly suggest not adding them until they are very close to full size chickens at about 4 - 5 months old. Some pen these growing chicks, next to the layer flock for a look but don't touch. Aart has a set up, which is rather like a one way gate, after a period of being next to each other, he puts this gate in place, where the smaller chicks can go in with the big girls, but can go through a gate that is too small for the big girls to follow them. Personally, I hate separate groups, and am for getting them in as soon as possible, as in I like the dynamics of a multi-generational flock, but I always use a broody hen which will just not fit your purposes.
 
As to your layers and roosters. The more of a closed flock that you have, the higher quality of birds you need. What would be considered imbreeding in people are common practices in chickens. You want to limit the genetic pool to the positive traits that you want. This produces a standard, and with a little luck, your birds will become more and more consistent with time. Most people on here are hobbiest. They are more interested in color of feather and eggs, and production is not a big issue. However, the standards of the breed have been established in order to maintain high quality birds that do meet production in eggs and weight with the least amount of cost. Over time, with a closed flock, if you do not pay attention to breeding, you will lose both egg production and dress weight of birds. But you are talking a long period of time, as in a decade. Maybe sooner, if you really start out with poor quality chicks.
 
Closed flocks are a worthy goal, but generally need some flexibility. Generally in the beginning of chicken raising, the idea of a closed flock is to keep disease out. However, it is an accepted practice, that one can incubate eggs from somewhere else or buy day old chicks from somewhere else and be safe. As closed flocks get older, more established and one is producing beautiful close to standards chickens, sometimes fresh blood can improve vigor of the chicks. But you are a very serious breeder at this point.
 
Mrs K