I wish to get a hair sample tested for hydrocodone/opiate abuse this is not for evidence but to encourage the subject to seek help. What is the absolute minimun sample required?
Also can I use DNA samples from relatives to cross check the hair to determine for sure that the hair is from the proper subject?

The DNA test would require a separate hair to test. But positive ID is done with a ID, fingerprinting or other method.
And why are you testing people you don't know??

But normally a hair is what you need, it can come from any part of the body, the head is normally used