Letter of Recommendation from Instructor at the Same University to Which I'm Applying

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Hello. I'm going to be applying for a PhD program at the same graduate school where I completed my masters. There was a graduate course I took during my masters program that I did very well in and which is related to the exact same PhD concentration that I'm applying to at the school. The course was taught by an instructor (not professor), who was a PhD student at the time and will be in their final year of the PhD when I apply. The instructor's PhD program (and even the concentration) is actually the one I'm going to be applying to, and I'm guessing their professors would be among the ones from which I'd have to choose an advisor. In fact, that instructor's advisor might even be one of my potential POIs, based on my research. (I'll be applying next fall, so I have some time before I start contacting anyone.)

Would it be acceptable to ask this instructor for a letter of recommendation? The school is one of the top three in this humanities field. I would be asking another professor from the same university (different program) and two professors from another university that I did prior coursework in, but I just wanted to see if this would be okay, even though the instructor who taught my course is not a professor yet.