Just logging on to say that I'd advise against mixing drugs with medication. SSRIs act by increasing the concentration of serotonin in the synapse. Psilocybin is metabolized into a serotonin receptor agonist meaning that it will enhance the activity of serotonin receptors just as your SSRIs are meant to do (although not in the same way). Maybe you could confirm by further research, but it would occur to me, at a first approximation, that shrooms could interfere with the activity of your medication.
SSRIs don't strictly directly enhance the activity of serotonin receptors - they just inhibit serotonin transporters, which shrooms don't touch, although you could argue that the net effect is the same. However it is true that there are a lot of reports of hallucinogens and SSRIs not being a great idea together due to overstimulation of serotonin receptors (from raised availability of serotonin in addition to psilocybin's effects on them) so it's probably not the safest idea ever. Lots of reports out there of nothing much at all happening too so it's probably not worth it, although I'm admittedly not altogether sure why it wouldn't work.
If you're going to make such a claim, it'd be best to back it up with some kind of explanation. In the absence of confirmation and the presence of higher than average risk (considering that you already have a high dosage of SSRIs affecting your serotonin pathways), I advise caution. This is definitely not professional advice, but a simple search on wikipedia would confirm what I'm saying here.
Spiff is entirely correct, and also should not be required to provide an explanation for something that's common knowledge among drug users. MDMA's effects are significantly lessened by SSRIs, and those taking SSRIs are also at far greater risk from serotonin syndrome when using MDMA. All-in-all a bad idea.
...not sure why you're trying to discredit someone's advice not to mix drugs, though.
edit: wait, are you referring to Spiff's advice re: shrooms or ecstasy here? If the former then disregard this bit.