@Nate raises very good points. My desire for Epicurus to be front and center is just that. A desire. However, if Raphael wanted to portray Epicurus, he most likely would either:
a) Use ridicule: pudgy wreathed portrait of guy he knew (and that resembles the Nuremberg Chronicle picture)
or
b) show him rejecting the accepted Philosophers: storming away down the steps or dismissively gesturing at the cynic Diogenes. Diogenes, with his Anti-social behavior and ridicule of Plato (plucked chicken = Behold, a man!) wouldn't have endeared him to the Popes but he's there. I see no reason why Epicurus shouldn't be included but he certainly didn't need to be.
Also, Lucretius's poem was also just being rediscovered around this time. "The first printed edition of De rerum natura was produced in Brescia, Lombardy, in 1473." Wikipedia. School of Athens was done 1509/11. Epicurus may have to have been addressed in the work, again via ridicule or rejecting accepted Philosophers.