Get letter from Post Doc or Professor PhD political science

[awfasano]

I have two of my three letter of recommendation accounted for and just asked a third professor that I did research with in 2016. He told me that the letter would be short and he didn't work with me in a Political Science way, which is true. I programmed something for him in mathematica from R. I thought this could help speak to some my quantitative methods and stuff but apparently not. He offered to go through some of my material to learn more about me, but this seems like not a great idea. I can't find too many Professors to write for me because I only took classes with three tenured professors in my political science classes and one was a massive lecture. Even my 4000 level classes were taught by TAs. I could ask a post doc that I worked with for several years. He was a graduate student while working on the project but now is a post doc. We worked on a project that led to a publication so i guess that is good. However, I am already getting a letter of rec from the professor responsible for this project, so I am not sure if its redundant to get him and the post doc to write letters for me.

edit: is a post doc okay to ask in political science is that okay? Or should I just stay with the guy I already asked?