This could be an interesting option although I think giving them a 7th man isn't gonna happen. Currently that slight reinforce reduction on the testing server is a nice addition but I do wonder how a bigger buff would help with that.
Maybe you could try giving penals a munition upgrade that gives them better dps to make it so they don't act exactly like 7 man cons? Cons would be the more durable and cost effective ones while Penals become harder hitting and better at attacking late game? The ATV could fit into that idea although idk how to exactly what the upgrade should give.
If i'm entirely honest, now that the door is opened by the Conscript seven-man upgrade, I don't see giving Penals a seven-man upgrade as entirely out of hand. Of course, there need not be an extra man added to the squad with a "mobilise reserves" upgrade, merely an upgrade in certain statistics. (Or the addition of ATVs, as previously mentioned)
If a seventh man did end up being a Commissar, he could well be given just a Nagant revolver. More a "Thematic" extra man, than a functional increase in firepower. It synergises well with "To the Last man", anyway.
Either way, i think it might be a nice area to explore, design-wise.