Have they been introduced to each other??
When the new dogs arrive, immediately take them for a walk through the new neighborhood BEFORE introducing them to your dogs. (Walking your dogs before they arrive is a good idea as well to burn off as much energy as possible.) The new dogs will receive a whole lot of information about their new home through their noses as they walk.
If possible, make the introductions through a fence first. You keep your dogs on a leash and enforce a sit or down if they get too excited. Excitement means jumping at the fence, barking, that sort of thing. All human handlers involved should look out for suddenly raised tails and ears, along with a too-long stare at another dog. That is a dominant dog ready to issue a challenge. Correct the behavior before it escalates to a fight by asking them to do something they know how to do - like sit. And stay until released. You need to work with your own dogs on this before the new ones come into the house. Matter of fact, all parties concerned should do a refresher course in obedience training now.
As soon as possible, start walking them all as a pack every day. You'll be surprised how walking together enforces the idea of pack cooperation. If two dogs in particular don't get on well at first, keep them on opposite sides of one another. Correct with a leash pop if one dog pays too much attention to the other. Fights can escalate quickly so you really have to learn to read dog body language to head off an argument before it starts.
Above all, the humans must remember to be pack leaders at all times. No rewarding bad behavior. Bad behavior means anything that a dog does that makes YOU do something you don't want to do - like barking all the time for a treat; or refusing to get off the furniture... that sort of thing. The less verbal your correction, the better. Generally a "no" and a firm touch at the neck to redirect attention is sufficient.
Other than that, buy an extra vacuum. :-D
Jaybay has a lot of good information. I just wanted to add a little trick I've picked up from fostering many dogs. I've got two dogs myself who can be less than welcoming of new dogs, especially when the new ones haven't been neutered yet and suddenly are here in our territory. During first introductions and for the first few hours I have the dogs together, I keep a full water bottle on hand with a squirt top. As soon as someone starts getting snippy or aggressive, I give them a good little squirt of water on their head. I might say a sharp "no!" but mostly I just let the water be the consequence for their action. Usually it snaps them out of the moment, and I can do it from a few feet away so that they aren't threatened by me coming to close or startled by me. (I worry that if I touch a dog that's focused on being aggressive, it might turn on me) Often afterward the dogs work out the pecking order quickly in more subtle ways. The dogs are well aware that I'm the one squirting them, and I think it just establishes quickly that I"m the leader and they're all my subordinates. Once that's understood by everyone, they quickly work out the rest of the rankings. Often there are little squabbles later but nothing vicious.
Great reminder mom! A squirt gun can be a pack leader's best friend - particularly when dealing with a true "pack." One trick a trainer friend of ours uses is to add about 15% white vinegar to the water for very stubborn dogs. The smell gets their attention when the water alone doesn't, and a mild white vinegar solution won't stain fabrics.
Thank you all so much for the great advice.