A couple general weapons notes:
1) In the early stages, defenses beat weapons as long as they are the same family. (Armor for guns, shields for beams, and AA for missiles) The balance shifts around mid-game, as the weapons get stronger and the hulls get able to mount more of them. One way to extend that period some is not to place engines in defender ships, and put defenses in the free-ed up spaces.
2) Keep track of what weapons the AIs are using. There are several ways to do it, including checking in the espionage screens, checking in the trade screen for weapons techs they might sell, and looking at the defenders around their planets. However, always confiem what the AIs are actually deploying by looking at their fleets out in space.