I foster greyhounds, much bigger than the little guys you're going to foster, but do introduce outside first on nuetral ground. This will let your dogs become more comfortable with a new dog and not feel invaded. Our fosters get crated at the very beginning. For greys it's calming to be in a crate in an unfamiliar inviroment. This also helps in potty training phase, dogs wont go where they lay. I also have cats so the dog is leashed around my cat until it shows no interest, usually right away since they are deemed cat safe when I get them. At feeding time, the fosters are fed last, after my two greys. They get their treats given to them last and my girls and the foster usually figure out who gets what bedto sleep on. They also don't get the benefits my girls get- (laying on the sofa or my bed.)This way the foster knows my dogs are the top dogs of the house-they are last in the pecking order. I also don't give treats to them without supervision, just in case of food aggression. And feeding time all three are fed in different areas so each one has their space of comfort. It is work to foster, not everyone can do it. It's a labor of love for these animals. We fostered a wonderful brindle boy for three weeks and he got adopted today. I loved him and feel a little hole in my life from him not being here, but I know he's going to a good home and gosh I can't keep all of them, but some do tug at the heart strings.

These are just some things I need to do in my situation with these dogs. Smaller dogs are probably different in alot of ways.
Good luck to you.