If you are going to foster then you need to ensure that you own cats are up to date with their innoculations.
Any foster cats you have in can bring things into the house. Its one of the reasons most of us who foster keep foster cats away from our own where possible (and at least for a few weeks). It's not fair to expect a rescue to pick up the tab for treating your own animals if it is something they can be protected from. Obviously, sometimes things like ringworm are harder to spot, and it is likely that this would be paid for by the rescue.
As far as neutering goes, if you cats are too young to be neutered then there is no problem, most fosterers have a mixture of ages.
Fostering is not for everyone. Some find it hard to cope with sick/old cats. Some people find it impossible to let the cats go. Some can't come to terms with the fact that some cats seem to stay for a long time. My longest term foster cats have been here nearly a year and they were 3 months old when they arrived!
