If I were to add a male to your flock of 10 month old pullets I'd add one at least their age, maybe as old as two years. An immature cockerel's hormones are usually running wild, he has little to no self-control, he often makes life hard on the flock, and it can be hard for you to watch. If you add a male that has outgrown that immature adolescent phase it is usually much easier on the flock and you.
Flock dynamics will change. I don't know how thy will change but here will be things different. I don't know what your facilities look like either. Adding an immature cockerel often requires more room than a mature male, plus the tighter I house them the more likely I am to have behavioral problems.
What are your goals for adding a male? The only reason you need a rooster is if you want fertile eggs. Everything else is personal preference. There is nothing wrong with personal preference, that can be pretty powerful. But it is a choice, not a need. I recommend you keep as few males as possible and still meet your goals. That's not because you are guaranteed problems with more roosters, just that problems are more likely. I don't know of the right number for you is zero or one. If you do get one, have a plan ready to immediately implement of it doesn't work out.
It appears as a white spot with a ring around it on the Yolks Surface...JJ
