Actually, mixing cannons and beams is not that bad.
If you attack at an angle you get very high dps, not quite escort level, but enough to kill things on occasion.
I have a dual beam, 2 cannons and a quantum torp in front of my sovereign refit.
The arc quantum torp, 2 beams and turret in the rear.
I use cannon spread fire, torp spread 1 and high yield 3 and tactical team plus tac captain powers to buff damage while having some survival abilities from the ship.
With all tac buffs and the cannon spread I can usually drop shields on my first pass and kill anything with less hull than a battleship/dreadnought with the torpedo high yield.
if a second torp hit is necessary, a spread from the rear torp can do it, since it only requires a minot course correction thanks to the 180° arc.