I'm going to be adding to our flock this September. The farmer didn't have any to sell earlier in the year (coyote attacks), but will have more about the same age as my current flock by then. The question is how many will you be adding, what age and what breeds?
Yes, quarrantine for 30days. Seperate house, seperate run, everything seperate and far enough away that they cannot sneeze/cough/poop upon other flock. Feed established flock first, THEN visit new flock - disinfect shoes - EVERY time! Different water containers, different feed containers - nothing should be the same between the two flocks!
After 30 days, some folks say put them together at night so they wake up together. Some say let them see one another for a few days, then introduce during free range time (so they're busy and distracted). However, all agree - they should be similar sized when introduced! (No introducing chicks to adults)
As the ones I'll be getting will be similar size (same breed - Black Javas), I'm crossing my fingers about introducing by the night-time method. I'm only planning on adding 2 or 3 maxium - to a flock of 6. So I'm hopeful it'll all work itself out. But I'm prepared to watch for a long while before I decide it's going to work.
It sounds like you'll be introducing one flock to another - so expect pecking order problems for at least a week (maybe more). Watch to make sure everyone has food and water, as well as putting in extra roosts and hiding places. I'd put out three times as many waterers and feed stations - just so no one has 'control' over one! And I'd rearrange their coop if possible or run if possible, so it's ALL new to everyone.
Good luck! Chicken math is fun, isn't it?!