My DS8's diagnoses is PDD-NOS, ADHD & GAD. My understanding is that the ASD/PDD-NOS diagnosis is primary by definition. The others are specified merely because the symptoms of them rise to a level that interferes with "normal" life, as it were.
As for treatment, your only option is to treat the symptoms. DS takes a med to help control his mood (Risperdal), and another to control his impulsiveness (tenex), and goes to one on one cognitive behavioral therapy, and just started group therapy. It was recommended he get pragmatic language therapy from an SLP as well, but we haven't found one yet.
I think it's just a matter of starting with whatever symptom/issue that's causing the most pain/trouble.
