Fighting pullets

At this age they are establishing "pecking order". It is perfectly normal and will most likely get better once they get older. She will probably just be head hen. As long as they don't get bloody or hurt i would stick it out. Introducing a new chick may not help, and your chicks may gang up on the newcomer. Hope things work out! 

There is such a thing as a pecking order, a little bullying is normal, shes letting her know who is boss, now if shes pecking her or drawing blood you have a problem.  She looks much larger then your BO at this point, but the BO will likely end up bigger in a few months, then the tables might turn. 

They are probably just establishing the pecking order. Started 2 of mine did it at that age, now there's more like four of them that chest bump and zoom at each other.sometimes 2 of them start and a third comes in. Most of the time they get along well and no real fights have broken out.  

Losing blood or feathers I would consider a real fight. Usually they just bump chests and stretch necks at each other. Last about 5 seconds at a time then it's over. Looks like they are trying to see who blinks first.

Hate to hijack but... I have been trying to introduce a BO to my flock of three.  I have a Golden Comet that is very aggressive and today attacked my BO because I didn't get up early enough to open the coop before they woke up.  She had bleeding from both her earlobes today.  My BO is the sweetest and our favorite and I hate that she is being bullied.  We also have a BR that will charge at her too.  It has been a month now.  They let her come in at night but she better be gone by morning.

My BO is super sweet and we have a BR that is very laid back and is fine with the BO (she is also the fattest).  But I have one Golden Comet that is very aggressive and a BR that is moderately aggressive, they both are very standoffish unless you have treats.  I am thinking if this continues we may have to turn them into meat birds.