I asked my vet about getting my cat vaccinated for FIV and he said that since she never goes outside she shouldn't be. He said that her risk of contracting it is non-existent if she isn't around another cat. Also, he said that if she should get lost, the shelter will test her for FIV and if she has had the vaccination, she may test positive. If what he said was true, it is possible that the strays don't have FIV, but may have been vaccinated for it. Unfortunately, there is no way to know for sure. If you have cats, keeping them inside is the best way to keep them safe.
I also agree with Muthoni that you shouldn't let the cats scratch or bite you because you could contract something else. If you do get scratched or bitten, wash the area well with soap and water to prevent infection.