need a little help with alien swarm mapping.

[jack254]

firstly i am making a map pack full of survival maps

for now i am trying to make an amphitheatre type map, but with a lava mote
i want the enemies to spawn in little rooms (8 rooms) they enter the map via drawbridge (that should retract after its empty)

but, what i realy want to do is have the rarer aliens spawn randomly (no parasites) how do i do this?