However, I would not try to add a single chick to an established group, even if they are the same size and there is plenty of space. A single bird is a very difficult to do. ALL of the original birds knows this is a strange bird, and they will all terrorize her. Either get more that a single chick (which spreads the pecking around) or separate one of your chicks now, and put with the new chick, separating them a few days with wire, then putting them together, when they get along, and they won't at first, then adding the pair back to the group. The group will still hand out some hard pecks, but with more to peck on it spreads it out so it is not quite so concentrated.
This is going to be pretty stressful for the whole deal I think.
Mrs k