First, thank you for taking this on. I've done some fostering, as well as some petsitting, although I'm sure there are many here with a lot more experience. Every situation has been different. Some dogs come in and fit in right away with little problem. Other times, we've had some serious jealousy issues with one or both of our furbutts. One thing I always do is keep them separated with a baby gate, etc., until everyone is on speaking terms.
Also, with the foster and all the additional work, its easy to forget about your own little guy, but give him extra attention, he needs to know he's not losing his place in the world.
Good luck! Send me a pm if you'd like to talk about any specific issues...I won't pretend to be an expert, but would be willing to try to help.