Most companions already use up ammo. Just give them a different weapon, not the one they start with. They'll only be able to use the other weapon if you supply them with ammo. As for other NPCs, tough to say. I've killed a ranger who was using a ranged weapon. I watched another ranger come ANC pick up his gun but only have like 2 or 3 rounds.
I'm sure there's a somewhat similar mod made to make enemies use more melee weapons. Most raiders run at me with baseball bats, swords, and pool cues. Where as NCR uses ranged weapons. And the Legion use almost only melee. However I do use mods for this. For the NCR I use NCR Rearmament, and for the legion I use Caesars New Regime. Both lore friendly, and it makes sense for both factions.
I could understand SOME merchants not having a lot of ammo. But places like GRA, Silver Rush, and NCR caravans craft and sell their own. I'm sure BoS scavenges and crafts most of their own ammunition. When it comes to energy weapons, I use the efficient recycling, or just convert from one type to another.
As for breaking things down and using them to make your own ammo, this is already a feature in the game. You can break down ammo types and make more using casings, powders, etc.
I'm not alright, I'm an equal amount of left.