Our policy with Siamese Rescue is keep them isolated for much more than one night. We keep them apart for 7-10 days.
There are very good reasons for this. First of all, transmission of disease.
And, longer isolation will make it easier on your resident cat.

Please, don't let any fosters in your home that have not tested NEG/NEG on the FeLV/FIV test and have the proof.

Cats coming from shelters are more often than not to have a URI. This takes a few days to manifest. Slowly, over the first 3-5 days, they start sneezing.

For the protection of your own cat, please consider isolation. They will get used to their room and not be overwhelmed with a new house, people and other pets.

I love fostering, it's an emotional ride.