I hear this a lot but the reality is in most cases you CANNOT do that. You cannot advance on Volks or Grens because they will pick your men off as they approach. Early in the game Volks and Grens are behind cover or their own sandbags as you're advancing.
Yes, again is really situational, as if you turn tables on volks or gren trying to take position while fighting against rifles whoms already in position can be the same thing. Is really depends on what situation you're going to put your rifleman or vise versa. Are they being supported with other infantry, are you bringing your own infantry, how many do they have vs how many do you have. What units within your hands can you use to counter their strat etc etc etc.
If lets say a volks squad alone behind green cover, and i have 2 rifleman needing to take over that position, 1 squad will take yellow or green cover and engage, while another will swoop in from another angle and attack it's position. Again is really depend on what given situation you're in and how you solve it, and what grounds you can move to make use of it.