Welcome to BYC!
Nah, you didn't screw up, but may have some management experiences ahead of you.
Adding any new birds can cause problems.
I assume the 11 month old (and maybe the 5 month old) is laying?
They might accept his advances, at close to 4 months old he'll probably start trying soon.
The younger girls, any not laying yet, will not appreciate his attentions.
Any young cockerel can be too aggressive and clumsy, depends on the bird.
They often will try to mate anything and everything, depends on the bird.
It really depends on the cockerel, how much space they all have.
Lots of space and places to 'hide' will make the situation more tolerable for all.
Always good to have a place to isolate an over aggressive bird if needed.
FYI.....semantics, maybe, but can be important communication terms when discussing chicken behavior.
Female chickens are called pullets until one year of age, then they are called hens.
Male chickens are called cockerels until one year of age, then they are called cocks(or cockbirds or roosters).