I would adopt her and quarantine her. After quarantine if your original pullets are still smaller than her just put her in and watch and see how it goes. Usually you only need to worry about the newcomer getting beat up by the existing flock, but your existing flock is still so young. The older hen will take her place as queen bee and I do not think there will be much of an issue.
When you put her in, make sure to watch carefully. If there is any type of penning into corners and serious floggings going on, pull her back out. Divide up ALL the birds into individual cages for about a week or two(keep them where they cannot see each other) and then throw everyone back into the coop at the same time. They will have forgotten each other and will have to restructure their whole pecking order. So tussling is to be expected but because they do not remember each other, there is no ganging up on any particular individuals.
-Kim
