I might be speaking from a place of survivor's bias, but this didn't seem to work against me this cycle. (For what it's worth, I'm pursuing a doctorate in Romance Languages and Literatures -- not English, Rhetoric or Comp Lit.) I had two letters from accomplished poets whose creative work engaged critically with the broad research topics I want to pursue. I was fortunate to have developed a close relationship with a well-known professor in my field during undergrad who was more than able to credibly fill in anything the other letters possibly could have lacked.
I do think if you go this route, my advice is to ask your two "uncredentialed" letter writers to speak to your intellectual appetites and aptitudes beyond the context of your creative writing, to the extent that they can do so honestly. Or to at least identify the academic strands in the work you've shown them.
Remember: there aren't hard and fast rules in graduate admissions, and anyone who tells you that any aspect of your dossier will be automatically disqualifying very rarely knows what they're talking about. I'd like to think your strongest case for admission is your most honest one.