Yeah I think if you do choose to combine these, it would be prudent to lower the dosage for both materials so you can get a feeling of how you'll react. I think kratom chemistry is pretty complex on it's own, but I've yet to hear of anyone having a bad interaction and i speak with dozens of individual kratom users daily. That said, there are so many alkaloids in the plant that it's hard to know exactly what is going to happen pharmacologically. There have been many cases of kratom interacting with other drugs that have lead to bad times and even deaths I believe.
I haven't read in depth but this paper may contain some information
https://journals.lww.com/ccmjournal...erotonin_syndrome_due_to_kratom_and.1286.aspx
I forget off the top of my head which alkaloids hit serotonin receptors, but I will say this. I think it would be less risky to use either pure mitragynine or 7-oh-mitragynine vs the plant. AFAIK neither of those alkaloids are interacting with serotonin receptors. If you're just trying to stave off withdrawal effects that might be your best option.
Also while you'll probably get more firsthand reports here, your post may* get more pharmacological specific responses in the Pharmacology and Neuroscience focus forum. Let me know if you'd like me to move it there after some time.