I hope that the foster mom is going to follow you home with the new dog in her vehicle, that's the safest and best way to do it. Once home you will likely have to let them get used to each other again just like in the parking lot before you bring them in the house.

I am more concerned once we get home with them all
If you're dogs sence you are nervous it won't help with the situation as they will pick up a scent from you and think you are affraid of the new dog which might make them react to it in a bad way. Try to be calm and relaxed.....I know it's easier said than done but if you don't things will be harder. hehe back atcha. From the email!