LOR requirements are school specific. Most require one from a pre med committee OR 2 from science professors/1 non science professor (or some similar combination) along with a DO/MD letter. I don't think a letter from an employer will count, but some schools may have a more lax LOR requirement. A letter from a pre-med advisor would probably be fine since it counts as faculty, but it would be better to get a letter from someone who has actually taught you.