Indeed, same here! It'd be weird if T3 is an M4203 and can be carried with but m79 couldn't. That'd make it completely redundant. Then again, we can have everything resist m4 ridiculously and weak to m79 so it's relevant again!
No, that was a joke don't do that.
As for being able to trash clear and kill bigs with those 2 and becoming a solo perk, I'd say your ability to do either should be 2 parts: The gun (foundation) and the perk skills (biggest modifiers). This way you focus in one or the other, even mix and match, but you cannot be beast in both. You lean towards one!
Current tree seems to support that well actually; Imagine the direct damage tree with m79+RPG. Your M79 trash-game wouldn't be as amazing as it could be but your RPG allows for the hardest hits with the M79 being a fallback/medium trash clear, which shouldn't require the extra damage. The AoE side and the reduction in damage would make the RPG a bit better at trash and less effective against big'uns, making them unable to 1-hit or 2-hit bigs!