Taking in adult birds from a different flock always produces risk. How you approach that risk depends on your flock keeping philosophies.
Some feel it doesn't matter as the birds are either healthy or not, can fight off diseases or not. However, that is risky...and it doesn't take into account the irritating transmissions of something like Northern Fowl Mite, which is a PAIN to have to eradicate once you bring it into the flock.
Most consider a two week quarantine to be the minimum with 4 week better.
But mind you, quarantine means that. Out of sight, out of draft, no shared fencing, and even further, you can't go from one pen to the next without some changes...at least in coat and shoes or you simply track in from one pen to the next.
Since true quarantine for disease is almost impossible in most small back yard holdings, it is easy to see why some just mix immediately as viruses carry on the wind and clothing.
If you can put them in the garage, that would be ideal. That way there is no wind transmission, and you can simply put on your house shoes or slippers to tend them in the garage.
Otherwise, you've got to put up a pen on the opposite side of the property, and do your best that things don't mix on the wind, through utensils/clothing, or through the ground.
I personally have an isolation pen (large dog cage) under the awning of my garage, which is on the side of the house in the front yard. There is a thick wooden fence between that and the backyard where my birds free range.
It is not ideal, but it is the best I can do (since hubby kicked all poultry out of his garage a number of years ago). I keep a separate coat and garden slippers by the back door that I go out of to just that isolation pen.
Be sure to keep different food dishes, water bowls, and at least change boots and coat. Tend to your regular flock first, then tend to the new birds. That will not likely prevent viral disease (that takes a hazmat tent or isolated room), but it will prevent lice, mites, and most bacterial infections being transmitted.
LofMc