Sorry you are having issues with your new birds. No, do not let them fight this out. Somebody will get injured badly or even turn up dead. You will need to keep the new birds separated for 3 to 4 weeks not only for quarantine and disease issues, but to allow the original flock to get to know these new birds. So keep your new hens in a cage or enclosure in your coop and run for 3 to 4 weeks. Everybody sees, no body touches. I have found one of those peck and play enclosures works well in the run and a large wire dog crate in the coop for sleeping with a roost bar, works well for sleeping at night.
Leave them this way and in a month you will know if anybody is sick and if not, the mixing will go much easier without all the aggression. There will still be some jostling but there shouldn't be any brawling. Of course if there is, always intervene if it turns bad.
Also, add more feed and watering stations after you do mix everybody together so that the original flock doesn't starve the new ones out, which they can do. Give everybody as much space as possible and keep to the 5 square foot per bird rule in the coop and 10 square feet per bird in the run or more.
As for telling whether or not a bird is sick...look for runny noses, watery or swollen eyes, rattling as she breathes, pale comb, lots of diarrhea, lethargy, blood in the poop, isn't eating or drinking, straining as if to lay an egg, mites or bugs, lots of broken feathers and red skin, etc...
Good luck with your new birds! Give them time to adjust to the new flock. Never throw in new birds to a flock as there is a pecking order that has to be adhered to at all times and chaos, blood and even death can occur without a long introduction of the new birds.
