I have no idea what you are actually asking or what your current situation is ...youre getting a silkie bantam?...you already have bantams..you dont say how many or coop size , youre introducing a pullet?? is that the cochin??? what are the ages of these birds...what is the actual question???? can you keep bantams and standards together: YES
Ideally they should be raised together as chicks.
If not then the new birds will need to live in an introduction cage within the existing coop for a week before you can start letting them integrate. After a week in the introduction cage, free range them daily as a group (new birds and existing birds) , it helps them to get acquainted without feeling so confined with eachother. And are you saying that when the standard cochin reaches full size that it will attack your bantams? The answer to that is no. They will establish a pecking order and adhere to it. I have a huge black Australorp living with a tiny little Millie Fleur hen and they are the best of friends.
Any chicks you buy , especially bantams will need to be at least 12-16 weeks old before you can even start thinking about introducing them to adult birds / again using the introduction cage method. ( some adult hens are more docile THAN others )