Well I think from Death's description, the Leviathan came before Eve. I guess I never thought she was ruling Purgatory, but more like the creator of monsters.
It is an interesting question that I'm not sure we'll get an answer to. I would guess though that since the Leviathans were strong enough to remain behind and also take over control of an angel (Cas) means they were stronger than Eve to begin with and most likely the ones in charge in Purgatory.
Of course it's just conjecture