What I do with the Galaxy build at least in PVE.
Dual beam bank, Dual heavy cannons, Dual heavy cannons, Hargh'peng Torpedo launcher
Since I bought the Sao Paulo, I swap out one of the DHC for the Quad Phaser cannons.
Normally if on a Galaxy-X I run Phasers on the over all ship build.
The Tactical powers I use are
LT Tactical BO powers.
Beam overload 1, Cannon Rapid fire 1
Tactical team 1.
LT science powers.
Science team 1, Tractor repulsors 1.
The Hargh'peng torpedo is a Fast, highly accurate, and does additional damage over time, plus that nice secondary explosion after 15 seconds. While the Beam over load, and cannon rapid fire allow you to unleash a LOT of power is short order.
IT is mainly a PVE build, but it is fairly effective, the Tractor repulsors help push most all enemies with save a few such as Borg cubes, AWAY from you and if used while in front of your ship, it will keep the enemy in your front line of fire. in line with those pretty cannons and dual beam banks.
Personally I wouldn't use the Galaxy-X for PVP. For the main reason, most Mobs you get teamed with don't know how to work with a Galaxy X, and they all go off and do their own thing, instead of attempting to stay together and work as a team. Unless they are in a fleet, using vent, or some other form of chat for co-ordination.