How dangerous is it to keep feral cats in my apartment with house cats?

Ask: The details are I have 4 feral kittens about six months old that I trapped last month, and they are in my garage. I am socializing them, and things are going pretty good. But naturally, they want to explore. So I have been letting them out of the garage and into the living room at night (I live in an apartment with an attached garage). 

My cats are being civil towards them, and except for some occasionally hissing from my cats, they mostly just stare at the other cats. The other cats are afraid of their own shadows because they are young and feral so my cats probably don't feel intimidated by them. My main worry is my cats catching something. 

My cats have not had any shots in the past 4 years. I made an appointment with the vet for my cats for next week. But I won't be able to bring the feral cats in til after the New Year. As long as I keep their food, water, and litter separate, and only occasionally let them out of the garage at night, will the risk to my cats be minimal? I am glad I got them out of the cold, but I worry about my kids (cats).

Answer:
HeartGold: I think you'll be fine. There is actually a lot of controversy about over vaccination. Vets started arbitrarily doing. Annual shots back in the 70s with no medical data at all supporting the frequent vaccinations. 

Jack: It is potentially very dangerous. Prior to letting them interact at all, the kittens needed to be tested for FeLV. It is highly contagious, and is fatal. Any viruses the kittens are harboring - calici, rhinovirus, or Panleukopenia - have now been introduced into your house for your cats to pick up. No cat nor kitten - especially strays - should be introduced to our existing pets until they are tested for FeLV and FIV, flea treated, wormed and given a clean bill of health from the vet. And your cats need to be brought up to date on their FVRCP vaccine as well. 

 Why would you risk the health of your cats by introducing them? Yes, you should be very worried. Keep them totally separate until the kittens see the vet and most importantly have been combo tested and negative for FeLV and FIV. 

Until that is done, keep them in the garage and totally separate until that is done. And hope that they are negative for FeLV, as it's fatal. Worse still it takes up to 90 days from exposure to the virus showing up in the bloodstream. If any of those kittens are positive you won't know if yours are infected for 90 days.