It is very common for the nursing litters to be cared for communally. It may be reasonable to test cats from unthrifty colonies. Certainly kittens removed for taming should be tested before being placed in new homes. There may be some increased fighting when new cats disrupt the local social structure. My primary recommendation would be to minimize relocation efforts as much as possible.
Untested colonies pose no more risks to companion animals than do free-roaming pet cats.
There is a common perception that feral cats have a higher rate of FeLV and FIV than do pets. Our research on more than 1,000 feral and pet cats in California and North Carolina, and published research in the veterinary literature show that this is not true. In reality, it is unneutered cats that wander, fight, and reproduce that are most likely to spread these diseases, regardless of whether they are in feral colonies or in private homes.