If your argument is that the plasma gun and fusion gun are the same thing, the obvious solution is to get rid of one of them, not make them even more similar but still keep them both.
The in-universe rationale has little to nothing to do with the survivor’s situation. Weapons manufacturers IRL love to make incompatible proprietary kinds of ammunition so they can insure a revenue stream from both the guns and the ammunition, but customers (including national militaries, which have a lot of weight to throw around) prefer standardized ammunition, so it’s a tension between the extremes.
In the case of exotic plasma weaponry, it’s a given that there hasn’t been an opportunity for standardization to occur, so it’s certain that there would be bespoke ammunition types from every company making a plasma gun.