Your first post stated the policy as "No facial hair that interfered with the mask seal".
When referencing 1910, we have to remember that the sealing surface actually varies on different types of masks. The posibility of interference with other parts also depends on the type of SCBA used.
So - I'd have to say you must PROVE his goatee is an obstruction. Personal preferences and "we always thought it meant" don't count if he decides to contest the decision.
That said, I'd rather side on caution but we don't always have a choice. Careful wording in policies is very important. So - can you get the policy changed...