Around here, "foster" is just code for "this dog's not leaving.
We always have a crate setup when we get a foster. We leave it open and they can go in and have their own space if they want. We also crate fosters if we have to leave the dogs alone together. I will not leave a foster dog unattended with our own dogs unless the foster is crated. There are too many unknowns with a new dog in the house.
We had one foster that didn't work out here. She turned out to be deaf which no one knew or picked up on while she was in transit to the group. We figured it out in a day or two, but she was very good at following cues and it appeared that she was responding to commands. What she was actually doing was watching Petunia and following her if we were going outside or called the dogs to eat, etc. After a week or so here, she attacked Petunia in an unprovoked attack when I was in the next room. It took quite a bit of effort, but I got her in the crate and away from Petunia. I was really glad that we had setup the crate. She ended up being the only foster that didn't stay here permanently.
That experience was the exception to how things normally go, but you need to take those precautions in case there is some reason that the foster just doesn't get along with your dog. 99% of the time, they wag tails and get along fine. All of the other dogs we fostered got along well and ended up living out their days with us.
Good luck!