My experience with crouching is that if you need to do a few things to keep them crouched:
setbehaviour "combat"
setunitpos "UP"
disableAI "move"
disableAI "target"
switchmove "crouch"
First off, disabling the movement AI seems to be the only way to keep them from standing back up (although I haven't tried looping or stacking switchmoves). I guess they end up trying to move a teeny bit, and that makes them stand back up. The setunitpos and setbehaviour seem to also help them stay in that position. Disabling the "target" AI is required, oddly enough. Without disabling the "target" AI, the units will actually MOVE and try to flank the enemy, or otherwise try to engage them. Yes, this even happens when their "move" AI has been disabled. The only member of the squad who stays put is the squad leader, so if you only have 1-man groups, this isn't really required.
And just as a side note, if you are trying to get units to shoot from behind some sort of cover (esp. sandbags), often you need to "setdammage 1" those objects, in order to help the AI realize they can shoot over them.