Maybe if you find a hen that needs to be re-homed you can try them out together, with less pressure on you. Just do the introductions the way they do for budgies. Each on their side of a cage wall, put the food bowls against the fence, once they get used to eating 'together' (face-to-face at the same time) the hard part is done. Eating together seems universal, it is what breaks barriers between strangers and enemies. If they get aggressive with each other, keep an eye, but let them work it out. Once the order is set, they will stop and move on to other things. Normally the fighting is very short, it is only an issue if it keeps happening, or becomes life/health threatening. Then it means something else than just pecking-order.