I. I do get rid of the dominant Roo 1st as usually it is a loud and mean one.
2. What breed are we talking about? Because that will determine Hen to Roo ratio.
People that have predator problems keep an extra Roo or two. Roos's that protect disappear or get themselves killed more often.
3. I normally cull the crowingist, meanist, hen chasing ornery Roos first. I find that they do change around in the pecking order, some that I thought wimps, suddenly standing out as gentlemen Roos. So I cull slowly if I can, just a few at a time, every other week or so when I want chicken for dinner.
4. I select by personalities, they must find food for the hens and let them eat first, they are the ones to woo the hens not ever chase them, they will dance for the girls, they will give warning to the hens and while the hens hide they stand out in the open crowing like crazy and protecting their flock, they will do this even as teenagers if you have a good Roo. Coloring and how pretty it is comes last in the choosing of a Roo.
5. I choose my Roo, I only have 1 or 2 at a time and they are very important to the girls protection and the continuation of the flock. I will take my children and Grandchildren's opinions under consideration or try to stir their opinion to the Roo that I am thinking of keeping, but in the end it is my choice. Besides, by culling slowly and spending a lot of time studying the Roos, even my choice changes.
6. To much inbreeding does lead to genetic problems and unhealthy chicks,(spraddle leg, croked toes, wry neck to name a few), down the line causing problems with a sustainable flock. That is why new blood lines are added from time to time, even on so called closed flocks. How often you add new blood lines is your decision, usually based on the health of the flock. If it doesn't matter if your flock is sustainable than you can pretty much ignore all information given on choosing a good Roo because any one of them will do the job of producing fertile eggs.
7. Let your Hens have a say in the choice. If you watch closely you will see several hanging out with one Roo or another (the one I thought a wimp turned out to be my best Roo). That is their choice, that I have learned to watch for.
8. Not every batch of chicks will produce a Roo that is an acceptable breeder.
It can be fun watching the flock dynamics change when selecting a Roo. Enjoy your flock.