Is it disadvantageous to have recommendation letters from postdoctoral fellows rather than professors?
You want the best letters you can get. A letter from a professor who doesn't know you well or your work will be worth very little as they can't honestly say much about you to boost your application. If a post-doc can, in fact, say positive things about you as a teacher, then that would be worth more. They are more experienced than you are and so will have some credibility. 

This is especially the case if they had a role in evaluating your teaching. 

For a more research oriented job the equation might work out differently, but for a specifically focused teaching job you probably would like one letter the can honestly and positively speak to that. The letter writer can include something about why it is they and not the prof who is writing. 

But most of the letters should come from professors.