It really depends on the harmala content of one's changa blend, as well as if that person has even added any harmalas at all. IME, the side effects/negative DDIs of harmalas seem to diminish when smoalked, along with duration as well as with varying concentrations. If only DMT is added, the choice of plants which one shouldnt add should depend on the toxicity/pharmacological properties of the plant itself (e.g. datura, poison Ivy, etc. wouldn't be very safe to smoalk, and thus shouldnt be added to any smoalking blend).
However, I have noticed that the choice of plant(s) in a blend of changa can have a profound impact on the characteristics of the psychoactive effect of the blend (i.e. the 'spirit' or feel) when smoalked. I found that a lemon balm heavy changa would be much moar unsettling on my gut and give me the feeling of a rat running around my intestines whilst at the same time amplifying the bright rainbow-y colors ad infinitum as well as adding a bit of a sharper edge to things. Caapi & salvia leaf (mixed at a 3:1 ratio) seems to produce a more pronounced fear/terror, hightened mindfuck as well as the propensity for a subject to have an OBE; plain ol' caapi leaf seems moar guided and cosmic without the weirdly intense salvia-ergic twist. The DMT seems to amplify the 'spirit' of the herbs used in the blend, and the harmalas seem to add a more grounding/guiding twist. IN my most recent blend (3:1 caapi:sally) of 2 g DMT: 1.5 g herbs:100mg harmine, the best effect of harmalas IMO (extension of the trip from 10-15 minutes to 40-50 minutes) is still present, however the grounding presence of the harmalas is minimized while the rainbow rollercoaster effect of DMT/salvinorin is much moar amplified, leading to a more chaotic albeit interesting/weird experience.
Back to the matter at hand, I would suggest that the OP use herbs which arent toxic to humans, and should place more emphasis on the DMT:herb:harmala ratio used in the blend, as, although the carrier herb seems to dictate the tone of the experience, the DMT:harmala ratio seems to have an even more profound effect on the general characteristics of the herbal-amplification effect of changa, and should thus be the primary issue to be addressed when designing one's changa blends. I'm still experimenting with various ratios; each different wonderful one i've tried to date has made for an experience all its own; i'd advise the OP to keep an open mind and to not be afraid to mix up the mess just to see the full range of the changadelic spectrum.
'"ALAS,"said the mouse, "the world is growing smaller every day. At the
beginning it was so big that I was afraid, I kept running and running, and I was glad
when at last I saw walls far away to the right and left, but these long walls have
narrowed so quickly that I am in the last chamber already, and there in the corner
stands the trap that I must run into." "You only need to change your direction," said
the cat, and ate it up.' --Franz Kafka