Would a Third rooster mess it all up?

[bops]

I would love to get another rooster, and now someone fairly local is selling =] I have a question though about fighting
I have an eight month old rooster (m sussex ) and a silkie, i dont know his age to be honest, but he is well past fertilizing eggs. They are both calm and dont fight. If i get this third guy will it stay the same do you think?
I have 12 hens, and may get another with the rooster, thanks!

 

[Percheron chick]

Adding any other chicken, hen or rooster, will change the dynamics of the flock so fighting should be expected. 12 hens is too few for 3 roosters too.