The bounds are easy enough to appy by limiting the platforms' Vertical movement within certain bounds. As for the enemies, they might have the 8 dir behaviour and their collisions disabled and put on another layer, but then they wont be able to follow the player in different platforms...
The thing is that in a 2.5D platform game the player should fall off the edge of the platform if no collision is detected and be able to jump on another higher platform as well if needed. How can this be applied to your example Tokinsom? That's realy an interesting topic:)