I love the creative use of your space! I would suggest some bantams though, if you haven't gotten the chickens yet. A few bantam cochins would do awesome in that space, and they can still lay a decent sized egg. OEGBs or Seramas or sebrights, etc would be even smaller birds.
I also think you could drill a few holes in the riser of your top step (along with the "green" area in that one picture).
I also think you could drill a few holes in the riser of your top step (along with the "green" area in that one picture).