Who can write the letter of recommendation

Hi everyone,

Nice to meet you and this is my first post here. I'm currently in the process of applying to multiple mfe programs, and I have an important question regarding the letters of recommendation. I've read through the websites of my targeted programs (cmu, Baruch, Columbia, UCB, nyu, etc). Although they didn't put any specific requirements about the recommender, I'm wondering if it's okay to ask for a letter from a PhD student or postdocs in the relevant fields.

I've secured two letters, one from a tenured professor, the other from a senior vp at the company I interned recently. My school has some pretty weird teaching stream as many of my courses are taught by PhD/postdocs, even advanced courses like PDEs or complex variables. I did try to reach out to some other professors for the 3rd letter, but unfortunately due to my negligence in the past, I wasn't able to establish a memorable connection with them. And they all suggested me to find someone else who knows me well.

I've mingled well with the those PhD student /postdocs instructors on the other hand, and they said they'll be more than happy to help me out. I'm confident they can say something solid about me.

However, I'm still concerned if this will impair my application as it's rare for a PhD/postdoc to write letter for masters programs.

Really need some advices and thank you very much in advance.
I would think a letter from a postdoc would potentially be okay, whereas a letter from a current PhD student would likely not be. That said, I would reach out to admissions at the programs you are applying to and ask.