Congrats on your new foster! Its a wonderful experience and you'll feel so great about helping this poor kitty.

Definately keep them separated until you at least know this kitty doesn't have any worms, bacteria, etc that could infect your cats. Also to give your cats time to adjust to the fact that a newbie is in the house.

Just go into the room your foster is as often as you can to give him/herl love, pets, food, playtime, etc. I have one girl who's been locked up in a room all her own since January. She'll never move out and thats a-ok to her (she HATES other cats) I simply visit her often and tell her I love her. She has her very own litterbox, food and water bowls.

Enjoy your first foster!