My experience is as follows:

• Missions that require capture of objectives are usually limited to 3 turns. You need quantity and some survivability to capture objectives
• Missions that require a model or group to be killed usually don’t have a time limit. You could feasibly just have 1 “killy” model, if your own target is survivable and you use your chaff well to block for them

So, my strategy is usually 1 Leader (also serves as backup killy / survivable model), 1 killy model, 1 survivable model, 1 of any other models I need to use specific abilities, rest all chaff.

Groups are split into Leader + chaff, killy model + chaff, survivable model + chaff