Pedroig, Smoelf:
You mean shots. Gauss ROF is always 5 (seconds).
"Gauss (1x6) ROF 5" is ONE gauss cannon that fires SIX shots every FIVE second interval.
Iceranger:
The fire control doesn't 'pick' -- the order is "fastest missiles move (largest salvo first), and this movement triggers Final Defensive Fire (if applicable), at which point the (numerically) first beam fire control fires all the shots of the (numerically) first beam weapon (or turret) and destroys (or not) X missiles. If any missiles remain in this salvo, then the next weapon/turret fires all of its shots. Repeat until no missiles (or no weapons) remain."
At which point the next largest salvo of that speed moves (or the largest salvo of the next fastest missiles) and the whole thing repeats until all missiles have moved.