Unfortunately. Yes. There is nothing in the rule book that says you can only have one problem. Being human is incredibly complicated and so are our illnesses.
Yes : ( (Unfortunately, there is no rule against having more than one diagnosis. Psychotherapy or even tapping are usually very effective in treating phobias.
ADHD & Phobias. Yes of course, although they are not related. ADHD symptoms include inattention, distractability, forgetfullness, procrastimation, etc. Phobias are associated with fear/anxiety. Both can be treated by a psychologist/psychiatrist.
Absolutely... In fact, phobias are a subset of anxiety disorders. It is known that about 30-40% of individuals with ADHD have an anxiety disorder. I would say in my experience though phobias specifically are more rare, but definitely happen. Phobic responses tend to develop from perceptual distortions and emotional overarousal. Both are issues known to be present in ADHD.