^It's physiologically safe-ish in that there's little reason to expect you're going to die or have any physical health crisis. There's a lot of confusion about serotonin and serotonin syndrome. Broadly speaking, serotonin syndrome is the result of there being far more serotonin in the brain and body than can be drawn back up or chewed up by natural enzymes before wild serotonin signalling makes shit go haywire. DXM does block the re-uptake of serotonin, meaning it prevents neurons from sucking the transmitter back into themselves, and results in there being more serotonin in the space between neurons. This alone is not usually of any particular risk (most anti-depressants, like Paxil, do the same thing). The most commonly encountered dangers are in combining serotonin releasers (which cause cells to pump out their serotonin) with re-uptake inhibitors OR in combing either of these with a drug that blocks the body's ability to produce enzymes that break up the serotonin like MAO (that's why taking MDMA [a releaser] with DXM or taking an MAO-inhibitor [like the non-DMT component of ayahuasca or some early developed strong anti-depressants] with DXM is dangerous). Most psychedelics (esp. a tryptamine like DMT, or LSD) largely just mimic serotonin, allowing them to bind with special serotonin receptors and cause a unique kind of signaling. There are exceptions like aMT, which is a strong serotonin releaser and is likely dangerous to combine with DXM.
To be totally accurate many psychedelics -- esp. phenethylamines (e.g. 2C-X) and psychedelic amphetamines (e.g. DOC) -- do have reuptake inhibiting properties, but for most people their degree of inhibition does not pose a health risk if combined with DXM (but they are riskier than combining it with DMT). As a rule, anything sold as a substitute for MDMA or meth is risky to combine with DXM because they most likely work by releasing and/or blocking the re-uptake of monoamines like serotonin (this most likely includes a long list of new legals like 6-APB, 4-FA, etc). People have combined them and lived or even liked it (then they recklessly post about it without qualification out of ignorance or a misguided sense of indignity at being told what they shouldn't do like their personal exception makes the rule, thereby perpetuating confusion and further accidents), it can't be denied, but others have died or had a horrific time. Never trust anecdotes when actual research and theory is accessible at your finger tips.There's going to be variable responses by person, dose, timing, etc. All that can be said for sure is that these combos vastly elevate risk.