For the Krait Mk. II? Three Pacifiers (L) and two beam lasers (M). Pacifier engineering is debatable, I prefer rapid fire/screening shell - but that eats through your ammo fast. Other people prefer overcharged. Similar DPS, but longer reload. For the lasers, efficient/thermal vent.
No experience with the FdL. For the Mamba (kind of streamlined FdL), I tried beams + Enforcers, but have now switched to a huge multicannon (overcharged incendiary), two Pacifiers (rapid fire/screening shell) and two Enforcers (high capacity, corrosive and emissive). That lets me run a smaller power distributor, which gives me some mass budget to fit better armour.
If you need the explanation: Pacifiers and Enforcers are both PowerPlay versions of conventional weapons - Pacifiers are improved Frag cannon, Enforcers are improved Multicannon. You get access to them by pledging to a Power for ~4 weeks and collecting enough merits from them. Engineering lets you emphasize certain aspects of a module, with corresponding penalties on others. Usually, the penalties are worth it. For example, overcharged increases the damage up to 70%, but costs more power from the distributor, increases thermal load and decreases the ammo clip size. Incendiary converts the MC kinetic damage to thermal (effectively turning the MC into a pulse laser, with lower power demands and limited ammo) at the cost of a massive increase in heat generation and a slight reduction in fire rate. And of course Engineering uses up materials, which you have to collect in the game, what a lot of people don't like (look for one of the "grind" threads over in dangerous discussions).