It is well documented that caapi leaf contains only traces of harmaline, so I don't think that this theory holds much water.

The mix of "inactive" herbs used as a base for the changa should also be considered for the potential of interactions.  Many commonly proposed changa herbs have all kinds of medicinal properties of themselves, some even acting as an SSRI.  Person to person differences in individual metabolism could cause differences in the strength of any such interactions.

Lastly, one should be aware that the solvent used to prepare the changa must be pure and evaporate cleanly.  Any nonvolatile contaminants present in the solvent will concentrate in the final changa mix (much more so than when freeze precipitating dmt during extractions.)