SSRI's tend to make psychedelics *slightly* less potent, but this can usually be compensated by increasing the dosage (however, if you do not know the dosage - and no, you can't trust your dealer on this - I would still just stick with a single blotter).
Seroquel, however, is a much bigger issue. Psychedelics work by activating the 5HT2A receptor, while Seroquel (and other antipsychotics) block it. Chances are you'll barely even get any visuals.