I think it is great that you are trying to follow the rules but I'm not sure you need to worry too much. Especially with kittens. Landlords put limits on the number of pets to prevent too many barking dogs (and complaining neighbors) and to keep pet damage to their units to a minimum. Since your neighbors probably won't complain and foster kittens are temporary, as long as you keep them in a space where they can't do too much damage when unsupervised, I think you can just go ahead with it.
I used to live in a trailer park for a while and had a mom dog and her litter of 8 puppies along with our two dogs and no one around me even knew!
#FosteringaPet