Q) Is there fall damage?
A) Yes there is falling damage, capped at 10HP
Q) Can nids fit through normal sized doorways?
A) No they are big, the tyranid in the MDK is the largest size, if you can walk around your map with that you're fine.
Q) are there control points/objectives?
A) Yes, use the ex_objective entity for control points and ex_mission entities to tell the players what they need to do to capture/control them. See the MDK documentation (
http://manual.40ksource.com/index.php5?title=Map_Development_Kit ) for more details.