it's honestly up to the individual persons body chemistry...some people who aren't on ssris also have problems tripping...hard heads..I tripped better on my 40mg prozac than when i was off....the trip was wayyy more euphoric and positive while on the ssri...but i'm not condoning it. I say it's 50/50 ...50% of people trip on ssris while the others cant or need a way higher dose. As far as serotonin syndrome is concerned...pretty sure that's a ongoing myth in terms of mushrooms...don't mix ssris with MDMA or maoi, etc as that can cause a deadly serotonin syndrome....I remember asking my psychiatrist if it was alright to mix the two and she said yes as long as i'm in a safe setting..but seriously don't take my advice as fact as you never know...just be safe and do your research/harm reduction
and ssris aren't serotonin inhibitors....they block the reuptake of serotonin so more serotonin is hanging out in the synapse and not getting 'recycled.' ..this in turn desensitizes your serotonin receptors and downregulates so you have less receptors (like a weed tolerance) so pretty much the psilocybin is in competition with the extra serotonin (which has a higher affinity) in the synapse which causes a 'tolerance'