Wow this
is an old thread!!!
Adding one chicken to an existing flock is fraught with risk...it's the hardest integration to attempt. If the rooster had been full grown and mature it might have worked.
Here's some notes I've taken on integration that I found to be very helpful.
See if any of them, or the links provided, might offer some tips that will assist you in your situation:
Integration of new chickens to flock.
Consider
medical quarantine.
Confine new birds within sight but physically segregated from older/existing birds for several weeks, so they can see and get used to each other but not physically interact.
Integrating new birds of equal size works best.
The more space, the better. Birds will peck to establish dominance, the pecked bird needs space to get away. As long as there's no blood drawn and/or new bird is not trapped/pinned down, let them work it out. Every time you interfere or remove new birds, they'll have to start the pecking order thing all over again.
Multiple feed/water stations. Dominance issues are most often carried out over sustenance, more stations lessens the frequency of that issue.
Places to hide out of line of sight and/or up and away from bully birds.
Read up on integration..... BYC advanced search>titles only>
integration
This is good place to start reading:
https://www.backyardchickens.com/a/adding-to-your-flock
