Check with State. Anybody can give advice about anything, but if you charge for it and make claims or imply that you are a health professional then you may run afoul of your states licensing requirements. Health professionals usually spend a great deal of time learning their profession and passing rigorous licensing standards in order to protect the public from scams or dangerous advice and treatments.
Depends on state. Some states license naturopaths, while others do not. The nest nutrition device may be gotten from a registered dietitians/nutritionists, who are always licensed. That said, vitamin shop clerks and owners will always give advice, but it should be taken with caution because they are trying to sell you something.