I find Plasma Slug Rail-Guns and Plasma Accelerators are fun to use in combination, but haven't tried the FdL yet. Heat and Distributor Draw will surely be the main limiting factors.
Use Efficient instead of Long range for the Plasma Accelerator. The longer range is not that helpful - the higher damage and lower heat, power and distro draw make more than up for the slower projectile speed, and falloff still starts only at 2km.
Maybe replace the fixed beams with turrets to free a fire button, even if this means much lower damage and cooling from them, or alternatively replace them with two more rails.
An alternative setup could be a huge, fixed, efficient thermal vent Beam and four rails or four PAs.
With plasma slug I always use premium synthesized ammo to compensate for the lower damage (with Plasma Slug the premium ammo persists when the ship is stored and through rebuys, but make sure to never ever use restock all nor individually restock the PA, avoid storing the modules, and don't trust the module section in the right HUD - it does not always show the premium ammo).
Since the FDL has only 8t fuel I'd install a second, larger fuel tank and go for a smaller HRP or MRP on the slot which now holds the Fuel Scoop.
