Is your foster coming directly from a shelter? Particularly a city (municipal) shelter? If so, I recommend a 1-2 week semi-quarantine period as a time for both a health and temperament evaluation. Sometimes they come with kennel cough (generally not a serious thing and I'd assume Tsavo is vaccinated which reduces chances of getting it or it being serious) and sometimes they have skin issues. Basically, keep them separate, provide separate water bowls and wash your hands between interacting with the dogs. Use different areas for potty. You should also watch for any issues like resource guarding or aggression or fear during this time.
If the dog is coming from a foster home or regular boarding facility and has had a vet check and at least some "known" personality traits, then I always do an on-leash meet-and-greet outside (park, city street) and then for the first 2-3 days I let the foster drag a leash around (on a harness) when the dogs are in the house together and I don't yet allow them to play in the yard together (I have two fenced sections so they can have yard time, just not loose together).
After that though, my rescue at least highly encourages plenty of supervised positive interaction between the fosters and the household dogs, assuming of course the dogs are good with that (both health and personality- for example if the foster dog has an injury or was recently spayed, they get separate time). It helps us learn more about the dogs, it gives them an energy outlet, it provides a good example (since most of our household dogs are fairly well trained and steady/confident dogs), and of course, it is simply less time consuming.
We don't leave the dogs loose and alone together ever though.
If you haven't read it, read the thread "Luna the foster pit bull" for a more informative look on one fostering experience.
Oh, and this
he basically let two small breeds (min pin and a chihauha/terrier mix) just jump all over him and bite his legs while he did his spin moves and puppy barks, making sure to not hurt either of them
is exactly how Chester is with small dogs. His whole play style and movement is completely different than with big, rowdy dogs (anything over about 40 lbs). He's had several TINY dogs run up to him and under him etc and the owners are always relieved to see how gentle he is. Keep up the good work on having him play/meet dogs of all sizes.