First of all, put them into Aware, not Combat - in Combat they will hit the dirt immediately, whether or not that's appropriate. In Aware they will only lie down if they really need to.
The best way to achieve this is through waypoints. Each patrol group of Soviets should have a bunch of move waypoints and a cycle waypoint to keep them going round their patrol route: when the Resistance are detected, a switch trigger should switch them into a new set of waypoints, appropriate for responding to an attack. The set of waypoints might look like this:-
move
move
move
cycle <----- synchro the trigger with this one
seek & destroy <---- set behaviour to aware in this one
cycle <---- you need this one otherwise they will eventually stop seeking and destroying
Use the field in the waypoint to set behaviour to whatever you want. There are tutes on switch triggers in the Ed Depot if you are not familiar with them.
And please read my sticky topic at the top of this board, which refers to this very question.