rwoodward

Always go with professors! I was given the advice to choose a professor I've taken 2 classes with over a research fellow (he has a PhD and a Postdoc) who works closely with me on my ongoing research. 
 
Also, if you believe that the other professors (not organic) know you better and will write you better letters, then go with them! I applied for organic programs this year and I've gotten letters from a P-Chem professor  and 2 Chem-Bio professors. I had one Organic prof that I was thinking about, but I felt that he wouldn't write me the best letter, so I didn't end up asking him. I got into all the top universities I applied to. 

My gut feeling is the following. I'd honestly say that you should rule out the research fellow from the consideration. What I've seen is that letters from folks who have a Ph.D. will carry more weight than those who don't. I'd do two professors you've done research with, and one professor who taught you.

It is best to include at least two people who have seen you do research. All the better that they're organic chemists too in your case. The third recommendation can be from any of your two teachers (better if they're well known in their field), doesn't matter if they're from a different field.
What matters is if you've had meaningful interaction with the recommenders and displayed your capabilities. The six month guideline need not be strictly adhered to.