We've fostered both cats and dogs in our house with 2 dogs and 5+ cats. Fostering of newborn kittens were easiest, the hardest one to integrate is our current foster cat. Fostering of goldens were relatively easy.

+1 with introducing the two dogs in neutral territory. At first feed the two dogs in separate place out of sight of each other. Last thing you need is have the two dogs "eye-ing" each other during feeding time. Once they sort out the pecking order between 2 dogs, then you can start feeding them together within eye-distance. later you can feed them next to each other. How fast depends on how well and how quickly the dogs will sort things out.

Beau and the three cats:

If Beau is the kind of dog that likes to chase (being a terrier), its best to keep Beau on leash when he is brought into the house. Also have a spray bottle ready! Both you and your DH should be reassuring the cat when your FIL brings in Beau into the house. Your cats are most likely bolt to go to their hiding place. Let them. Cats are curious enough so that when the dust settles down, they will venture out.

One thing to remember, your cats may show their disapproval of Beau by spraying around the house so have "Natures Miracle" ready to clean up the spray.

Squirt Beau with water if he shows any inclination to chase the cats. And keep an eye on your cats. After about a week or so, the cats may get enough nerve to attack the dog in the face.

The whole integration time could be short as few days and may be long as a month.

MAKE SURE SOME ONE IS AROUND TO SUPERVISE AT ALL TIMES FOR THE FIRST FEW DAYS. You, your DH and FIL will also need to readjust with the animal dynamics. And keep a close watch on their health. Stress of new animal in the household can cause some illness. Diahhrea is common.

When all the animals fall into their new routine, you'll forget about the stress of integration.

enjoy the new dog in your household and your FIL too .