I was in a similar position and when I asked for advice from others in the field (I'm in Psych) I was told that if I could not get all of my letters from academic professors or research supervisors, then letters from employment or volunteer work would be fine if they were in my intended field. I too have supervisors that have varying degrees of education and I was told to select those that can best speak to my skills, abilities, professionalism, and capacity to succeed in grad school. I was also told to select those that had the highest education whenever possible. So a supervisor with a PhD or MA/MS would be great. A supervisor with a BA/BS or classified as a graduate intern (as in their degree wasn't quite finished) would not be ideal.