Why should a mining rig necessarily have energy weapons? Or weapons at all?
The missiles could well be mining ordnance, designed to penetrate hard rock. Sure, death rays could also be used for the purpose, and Nero has a really big one for exactly this - but having the big beam can be taken both as indication for and counter-indication against the existence of smaller beams.
In any case, we never saw any sort of secondary weaponry, beam or otherwise. If Nero says "Fire everything!" and all we see is missiles, then that's pretty good evidence that the missiles are everything he has.