The aliens having room temperature superconductors would help explain their particle beam weaponry. The energy requirements for accelerating even sub-atomic particles to c-fractional velocities is non-trivial. A superconductor that doesn't need to be super-cooled would allow for high energy density storage which in turn would make it possible to power a pocket particle accelerator.
Regarding the plasma thing, the more minimalist the explanation the better. The weapon is implausible as is so we might as well just put in a short explanation to minimize hand-waving. Say "the plasma is encased in a bubble of a magnetically charged material" and leave it at that. The short range is explained by either the material (or plastic if you like) being stable for only a short about of time, or it rapidly losing its charge. Going about explaining just exactly what the material is, and how it is magnetically charged, or even saying the word "superconductor", is asking for the death of the audience's suspension of disbelief.
It's a good rule of thumb to minimize handwavium whenever possible.