Mixing a new bantam with larger older girls

She will be picked on for the rest of her life.  Silkies because of their smaller size and 'different' look frequently become targets when introduced to an established flock.  When raised together from chicks, bantams and standards can get along fine.  I would be concerned about integrating a single silky into an established flock of standard sized fowl.  Chicken society can be very cruel.

There is a slightly better chance when introducing multiples to a flock - the provision of hiding places might also help.  There will still be pecking and attempts at subordination.  Your present birds are not fighting because the pecking order has been set and no one is regarded as a 'stranger'.  Think 'mean girls' in high school.  The safest scenario would be a couple of silkies in a separate pen.

Here is from my experience i got 20 chicks all only a few days old. Month later added 2 silkies one ended up being a roo and the only reason his sister didn't get picked on. The hens did not like them at all. the roo though took charge and protected his sister from the hens and also beat up the hens. They had to get separated. They now have their own coop. I believe if we had gotten them a few days in between or the same day we wouldn't have any problems, but because of the gap it didn't work and if both silkies had been female we would have had even more problems.

You could keep it in the house, or you could build a small tractor or smaller coop to house it then you could get two or more silkies XD