I'm missing two gallery rooms.
(...)
I'm pretty sure I'm missing the Gargoyle, but I have no Idea where they spawn.
Certainly not in the soul prison. I went there dozens of times, and it was always beholder and prisoners only.
Prisoners spawn regardless as they are obstacles like the changing labyrinth in the desert and the swamp water slowing you down.
Some stages/labyrinths have separate enemies on repeat playthroughs, it is chance based who you get.
Here is who spawns where from what I recall:
Direworlf/Arachne - Dungeon
Harpy/Chamaeleon - Path of Belief
Swamp Stalker - Swamp of Death
Lamia Guard/Sandwitch - Forbidden Desert
Nightmare (weeping angel statues) - Dark Forest
Gargoyle/Beholder - Soul Prison
Wererabbit/Mogu-ring - Lunatic Night
Kitsune/Ao Oni - Chaos Realm