Jirishman77,
Note that the Rambam doesn’t say “Rabbi” in his list of things a Ger cannot be appointed for. Geirim, according to the Rambam’s understanding, can not be given any positions of actual power. But this does prevent the Ger from being in a position of persuasion. A Rabbi, as opposed to a Dayan, has no actual authority, no actual power. He can’t enforce his positions or rulings anymore than the average person.