Hello all, I have about 18 regular size chickens in our run but want to add bantams to our flock for my son for 4H. Is there a ratio we should have bantams vs regular to keep them from getting picked on because they are smaller?
The short answer to your question is there is no magic ratio anytime you integrate any chickens. There also is no magic minimum number that will stop all aggression other than that trying to add just one is especially risky. You need at least two.
Age is pretty important. A flock of 18 mature bantams would probably pick on new immature full-sized-breed chicks. Size isn't that critical but maturity is. In 4-H you are probably starting with chicks so they will be at risk. As Brahma mentioned, his bantam hen is dominant over full-sized hens. It's not the size of the chicken in the fight, it's the size of the fight in the chicken. Immature chickens don't have the fight in them that mature chickens do.
The more room you have the easier an integration usually is. One way chickens have learned to live together in a flock is that when there is conflict the weaker runs away. There may be some chasing but as long as they have enough room to get away it usually ends without any getting hurt. If they don't have enough room to get away the winner does not recognize they won and keeps attacking. This can lead to severely injured or dead chickens. This is amplified with immature chickens as mature chickens often attack immature chickens that invade their personal space. They need enough room to avoid the more mature. The quality of the room can be important too. It is better to have places they can hide under, behind, or over than just a bare open space.
Some people have no problems with this but some do, especially when adding new chickens. Some chickens get their feathers plucked if they look different. A common example are Polish with their head-dresses. It's not because they are bantam but because they are different. It doesn't always happen but it can. If they are to be shown for 4-H you don't want their head-dresses plucked off.
Even if I knew a lot more on your set-up, conditions, and what integration techniques you plan to use I could not give you any guarantees on how it would go. Sometimes these things go so smoothly you wonder what all the worry and warnings were about even if you don't follow all suggestions. You may try it and have no issues at all. Or you could have a disaster.
My suggestion is to try it but have a Plan B ready to go. Plan B is probably a separate pen/coop/run where you can house them immediately if a need arises. Hopefully it will be where they can all see each other and you can try again later if it doesn't work out at first.