Mirtazapine is definitely much different than other proserotonergic antidepressants in that it blocks serotonin at two specific binding sites (5HT2 and 5HT3). It also blocks other serotonin sites, but pretty weakly by comparison (its 500 times weaker at 5HT1). This strong blockade at those specific sites can can cause it to "spill over" in excessive amounts to other sites (5HT1 specifically) which could result in serotonin syndrome. This almost never happens though, and when it does, its almost exclusively in the elderly.
Serotonin syndrome caused by mirtazapine is many times rarer than with other antipressants. Because it is a strong antiserotonergic drug you could potentially use it to treat serotonin syndrome in a pinch (if you had no alternative, definitely not the first choice for serotonin syndrome). If i was in the middle of the desert with someone suffering from serotonin syndrome caused by MDMA for instance, and I had mirtazapine on me, I'd give them mirtazapine, probably at least 45mg chewed or more.
It is unlikely to result in serotonin syndrome in a healthy adult, even in concert with stimulants. I myself have taken it with MDMA at high doses. Propylhexedrine is surprisingly serotonergic at high dose, but not nearly as much as MDMA. Who knows, take care and I'd recommend you don't exceed 500mg of propylhexedrine
