Recently my horse Sass was introduced into a new herd. Apparently, she moved into the top mare position and the other horses respect her needs. Surprising since this horse seem to so mild and will be used to train children in horse riding. She also won a few awards and loves to trot and canter. Might be one of the reasons why she came out on top.
I will say that there is a way in which one should introduce a horse to a herd. Remember that horses are creates of hierarchy (like we humans aren't) and will kick, bit, and fit a little to create a social order after a few hours or days. Make sure you do a few things that can help lessen the issue.
1.) Separate the new horses into paddocks but let them introduce each other.
2.) Feed all the horses before introduction to keep them calm.
3.) Walk around the yard to show the horse where it will be staying.
4.) Remove places where horses can get trapped in scuffle.
5.) Watch the horses for a while to ensure they are not getting excessive.
Here is a really good article. https://equusmagazine.com/behavior/newhorse_032006-8162