If I can find anything related to such made by Valve in the Source engine, I'll try and make one. However, as far as I'm aware, I'd need to create some sort of custom AI system for NPCs like Combine Soldiers to inherit the defensiveness of the HECU grunts instead of just their aggressiveness (in which the metrocops kept the defensiveness) and to also implement your suppressing fire suggestion. The closest thing to switching up Combine's tactics is a value in their entity properties, which goes as follows;

Normal Tactics, which are their standard strategies;
Pressure the enemy, which will force them to push forward;
Pressure the enemy until 30ft, then normal, which will force them to push forward until they reach a certain distance.

The tactics cannot really answer your suggestion unfortunately, but they're probably the closest thing to what you're after in terms of different tactics.