I add fosters into my pack all the time, and they have a wide range of temperments. I must say that I might go overboard for some, but it works for us and we haven't had an ER visit yet(due to that).. *knock on wood*
I don't like to introduce them right away to each other, instead I play swingshift for at least a day to get them used to each other's scents. I have them potty in the same area, so that they can sniff each other's scents outside as well. Then I'll slowly introduce them one on one, with the foster being on a loose leash inside(because I know that my guys will listen to me to back off or whatnot). I can usually tell right away by the body language if I should cut it short and wait a while longer.
After they have met for awhile, I'll integrate them into the pack/household for longer periods(and make sure to drag the leash when they're out, JIC). This worked extremely well on my first foster; a crazy untrained, older, recently neutered, outside lab/goldenx named Max. So I've used it to varying degrees with other dogs who've stayed with us for long periods.
Most recently I've used it on "evil wiggles", my mom's chow/gsd, who is staying here until she finds a new home. My guys "knew" her because she visited a lot, but things were chaotic when she came to live here since Wiggles has a very dominant personality. She also has a high prey drive and went after Kida almost every chance she could. Now, after the slow introduction period(and a couple of grabs on Wiggles part), they all have free reign together and they all sleep loose inside. So, it worked out nicely.
Good luck on finding the right new addition to your home. I'm sure the perfect one will come along soon.