My idea: As they stand, ships look woefully unarmed, but perhaps with these they could be a bit more pleasing to the eye.
The rave gun is fluff, plain and simple. Its there too look cool, kinda like spinnaz or under-car lights these guns would be added on to whatever and wherever the devs would want to put them. They do no (or next to no) damage, have a VERY small cool-down, fire medium distances, come in a variety of preety colours for different races/ships.
Rave guns would be composed of 1 single emiter and a small trail. "Rave lazers" could be replaced by whatever you (the devs) decide.
Purpose: to make it look like the ship is FULL of weaponry, while maintaining balance.
Possible problems:
rave lazers may be confused with other projectiles
Fix: Have the projectiles small and distinctive, different than all the others.
Lag issues
Fix: if lag does become a problem, raise the cooldown on the lazers making them semiautomatic or even slower to fire, if the problem persists lower the range of the lazers, if it still continues either remake the particles or scrap the turrets from some ships.
Where and how many ?
The max amount of turrets should be around 6, that said they should be small, hard to notice when not firing, but there nonethe less.
most ships would work well with 2 or 4, if the ship is a frigate they should be limited to 1 due to lag issues.
rave cannons should be locaded on the sides of a hull to emulate broadside barrages.
If all else fails:
Conclusion: these cannons would not effect balance or gameplay peformance if carried out or made in the proper way, should all else fail, simply make them actual weapons, that shoot slower than autocannons and do medium damage to fighters and frigates.
Other Possibilites/turrets:
Minigun turret
*fake* torpedo launcher
quad cannon lazer/rail launcher
*fake* spam/spaz missile launcher