I'd like to say you could get a pullet that will never be mean to others. Some breeds have reputations, but in the end there are personality variations. I believe that hens need at least 3 to cause issues, if there is only 1 other bird with them, there is no way to "gang up", they must be friends with the other bird or they will have no friends. So, if you choose to separate her, one of your existing pullets might befriend her if it is only the 2 of them. Of course, they might both choose to be broody together . . .
X2
We often choose a mellower personality hen or pullet to keep injury recovery birds company, broodies don't care if they have company but keeping any bird out of the flock for extended periods can cause reintegration issues. I have a few 'go to' company birds and a couple I avoid because they tend to be bossy and pecky.