I haven't read the module in question, so I don't know if it addresses this, but "typically" when it comes to planar creatures being in particular rooms, it's because someone has summoned and bound them there to act as guardians. So they never traversed the too-small corridors to begin with (nor will they try to leave the area, since they can't).
Likewise, even leaving aside that slaad are chaotic in the extreme, creatures bound to a particular room are (as I see it) either going to try and keep everyone out as a matter of obeying the magic binding them (and so will shut the door, bar it, ready attacks if it opens, etc.), or will be desperate to go home, which includes dying on the Material Plane and reforming back in their native realm (typically with the proviso that they can't kill themselves/each other), and so will initiate combat at the drop of a hat, even if it's not advantageous to do so.