Considering you can only have one spinal mounted weapon, secondaries will likely make up most of your firepower.
What I do when designing beam ships is decide on a total tonnage and speed. This tells me how much tonnage I can spend on weaponry. Once I know my weaponry budget, I put in the biggest spinal weapon I can, as many gauss turrets as I think I may need, and then I fill the rest of the space with the biggest normal mount lasers I can build. You don't need to turret them, with their low rate of fire they'll be bad against missiles no matter what, and if you're in a beam fight against someone who vastly outpaces you you're already dead and no amount of tracking speed will save you.
So in a 6000 ton beam destroyer, I will have one spinal laser, 4-6 normal lasers, and 2-3 gauss turrets.