Depending on the personality and breeds of the large flock it can be extremely difficult to add a single bird.
The more space available and extra feeders/waterers help.
Rule of thumb is to introduce like numbers and like sizes.
If you have more housing, you can place one of your birds with the new one for a day or two, and every day add another bird till you have a good size flock in there, then introduce the birds.
This is also a good way to quarantine. If the new bird is bringing in disease, you'll only lose one or two birds.