Stop worrying
True rage is very rare in cockers but there seems to be a tendency for people who do not know the breed (including some vets
) to talk about rage when they are just talking about normal puppy behaviour
There
are cases of rage and, you are right it is often down to bad breeding, but they are very unusual.
Is there any particular reason why you have been researching this?
If not, just relax (as much as you can with a puppy in the house) and enjoy your new addition. Someone will be along soon who knows more about this than me and I am sure they will put your mind at rest.
Oh and we need more details of your pup and some pictures - they are compulsory on this forum