A single bird addition can be tough, but if I read your post correctly, you only have one bird. What I would do is pull your original bird out of your coop run set up. Put your new bird in there for a couple of hours by herself so she figure it out, eat and drink, then just before dark add back your original. There may be a dust up, but let them work it out unless there is blood. Do it as near dark as possible, as the urge to roost will be nearly as strong as the urge to fight.
Make the addition on a day before you are going to be there the next day and keep an eye on them.
MrsK