Hey man, med student here. You really should not take shrooms while taking a SSRI. Psilocybin emulates the serotonin molecule, acting as a direct serotonin agonist. SSRI:s, like Prozac, are serotonin reuptake inhibitors. Serotonin is, kind of, circulated in the synapses, released and reabsorbed on the other side as a way of relaying a neurochemical message between nerve cells. SSRI:s prevents this reuptake of the neurotransmittor, in this case serotonin.

http://en.wikipedia.org/wiki/Selective_serotonin_reuptake_inhibitor

When you have a direct agonist releasing serotonin into the synaptic clyft, and a SSRI preventing the reuptake on the other side, this can produce ridiculously high serotonin concentrations in the synapses. THIS IS MASSIVELY DANGEROUS! It can lead to something called Serotonin syndrome, which is an acute - possibly fatal - medical condition.

http://en.wikipedia.org/wiki/Serotonin_syndrome

TLDR; Don't mix shroomies and teh SSRI