Hello,
In v2.5, producing Plasma pistol and Plasma rifle is possible as soon as you complete their respective research.
Furthermore, you don't have to research Alien materials to build those weapons though they need alien materials scrapped from UFOs.
I figure something is weird, and because of the later, I'd think that plasma weapons are not meant to be produced by PHALANX.
Suggestion: don't allow the production of alien weaponry (or modify the prerequisite tree).