Very nice concept and well executed so far. This could definitely be expanded and be a great puzzle game.
One thing I noticed though is, unless I'm missing something later on, the flying monster seems to be a bit overpowered since it can both move over pits as well as exit the level. There were multiple times that once I got the flying enemy I didn't feel the need to switch to anything else.
Perhaps one way to balance this would be to remove the ability to move between floors with it, but leave everything else. That way it could be used more strategically as a means of traversing the level but primarily as a means to reach other monsters (which seems to have been the original intention anyway).
Aside from that the only other issue I ran into was the looping music cut out between each loop.
Great game so far though!