1) It's dependant on the gravity (which can be different in different levels) actually. At default, I believe it's roughly a 2:1 ratio of height-to-width is unpassable, but a 1:1 ratio is, I forget where in there exactly it becomes impassable.
2) Nothing special. To make them MUCH more alien-friendly though I'd recommend applying a 'ramp' under the stairs. and making the stairs themselves illusionary. I.E. Non-solid, players/bugs walk on the ramp underneath, buildings get played at the 'angle' of the stairs, but the stairs look normal. If the stairs are exceptionally broad, shallow ones (4:1 width:height or more) the above is far less required, but remember that wallclimbing aliens may not want to turn off wallclimb just to go across the floor, and their view gets jerked around and they get slowed by by every angle they have to cross since they loose the 'stairclimb' logic to hug the walls.
3) Dunno, it's standard Q3A height.