Visually, placing figures in close proximity is an easy way to demonstrate familiar or intimacy, the prime example of this being the famous teacher-student relationship between Plato and Aristotle, which is the first image to which the eye is drawn.
If this trend were consistent, and Raphael had an understanding of Epicurus' biography, then we'd expect him to be placed nearest to Democritus, followed by Aristotle (I doubt that either Praxiphanes or Nausiphanes would have made the final cut).
He may also have placed him near Pyrrho, ONLY because multiple sources mention Epicurus as having admired or been fascinated by Pyrrho and his Eastern journey, even if he completely disagreed within his findings. I think this is more of a stretch.
If proximity isn't indicating relational intimacy, it may be expressing ideological tension and conflict, in which case, Epicurus should be placed near Plato, and painted in a critical, dismissive, or challenging position, which we do observe in the figure in orange to the left. Scholarship online seems to place Heraclitus and Parmenides in close proximity downstage of Plato and Aristotle, so that further indicates an expression of tension. Though, I'm not convinced of those identities. Still, placement near Plato is appropriate.