In one of my previous postings I stated a "Golden Rule" that I used when bring new foster babies into the household... LEAVE THEM ALONE... briefly...
#1 I bring the newcomers into the home in a carrier, leave them in it, on the floor where the others can approach and sniff. If there's hissing just ignore it. I've found that after 20-30 minutes everyone calms down enough to open the carrier door...
#2 open the door and LEAVE THEM ALONE... the newbies will try to explore the new surroundings and the resident kitties. Unless you have a cat fight involving blood (which I have NEVER had happen) after fostering over 30 cats... LEAVE THEM ALONE...
#3 make sure your resident kitties don't feel left out for the new ones... give them extra love, but if they are interacting with eachother .. LEAVE THEM ALONE.....
#4 make sure that there are full dishes of food and water, add an extra dish for each newcomer.... this reduces competition for food.. and LEAVE THEM ALONE if there's hissing at feeding time....
#5 when everything is calm enough to pick up the newbies, show them the litter boxes (if they haven't already found them) and then LEAVE THEM ALONE...
#6 I can't stress this enough, no matter how much you want to "Help" them LEAVE THEM ALONE..
#7 The only thing I do it talk to and reassure the resident kittie or kitties. Tell them they are ok, and good kitty and LEAVE THEM ALONE....
#8 Did I mention the most important thing of all... oh yeah see #1 thru 7 LEAVE THEM ALONE....
I've never had it take any longer than 3 or 4 hours before the hissing stops and everyone finds a place to relax ... soooo LTA