1) The range for dimensional gates should probably be 5 to 30 like the totems, although I would guess that 5 to 100 ends up working the same way.
2) Their spawn chance will be ignored in levels that they can't spawn in.
3) It does pick a few initial monster types and sticks to just those types unless a quest forces a new type to be spawned. The reason that you also get things like MonsterFog is because they are specified using ObjectSpawn instead of MonsterSpawn in world.gdb or world2.gdb.
The scree will be fixed in 1.025. I guess everyone gets a new (kind of) monster in the next patch.
Depths of Peril, Kivi's Underworld, Din's Curse, Drox Operative, Zombasite, Din's Legacy, & Drox Operative 2