On page 10 of the rulebook it states that a piece can placed regardless of where it is as long as the piece could otherwise occupy the space. I'm assuming this means so long as there are no other pieces there that would prevent a piece being there e.g. Two enemy pieces in one location. But does that also extend to a piece requiring supply?
No, being in supply is an additional requirement for mustering pieces. A piece that is out of supply will be removed during the following supply step though.
Additionally can an enemy hoplite share a location with a friendly city? I.e. Can a spartan hoplite be in Athens while an Athenian city is in Athens?
Page 10: You can never place or muster a piece in a space occupied by an enemy piece.