Anecdotally, I can offer my experience.
I have coadministered kratom and Harmala/caapi many times, especially lately as my daily level of pain requires me to use an opioid at least once a day (and with that I am still in quite a bit of pain- usually it is 2-3 times daily). I don't LIKE to do it, not for any fear of health problems, but rather I feel that it blunts the experience and leaves me with less of an Impact. This could be psychological.
I have also taken Harmala with 10mg hydrocodone (prescribed of course), with a similar experience as with kratom. I feel it takes away from the experience. But, then again, so does being wracked with pain. So I feel a little like my hands are tied. It's lame, I don't even like taking these things daily AT ALL and I pray one day I won't need them.
And finally I once took 5mg oxycodone at the end of a Jurema/Harmala experience, for pain, and had no negative consequences.
However of course YMMV and its better to not mix them, just for the sake of the experience