I was thinking that might be the case but I never tried it (enemies generally seem to be able to pass through each other). In my specific case I wanted to use it for a certain boss though, and it seems like having any other enemy in the room scews up the graphics. It doesn't have anything to do with allowed enemies or something like that... it wasn't even on the same screen. I did find a good alternative though so it's not really a problem, just asking out of curiosity.
