Machine Gunner squads retained from the Pacific campaign and the Battle of Stalingrad have an upgrade option in their squad upgrade screen that allows an additional Assaulter to be selected. However, after the campaign merge, all three German Machine Gunner squads, as well as some US and Soviet Machine Gunner squads, no longer have this extra option. I believe this option should be uniformly added to all Machine Gunner squads — just as the three Japanese Machine Gunner squads inherited from the Pacific campaign all have it.
Definitely. An extra assaulter is critical at low BRs, as it can markedly enhance the fire density of Machine Gunner squads. For these squads in low BRs, suppressive fire is everything, and only three soldiers with automatic weapons are often not enough to maintain it.