Considering that the Defiant model was built without paying attention to how the standard pieces of TNG-era technology normally look, it's amazing how consistently the VFX people were able to associate a specific effect with a specific location and a specific piece of dialogue.
Every time the ship is seen firing the white-hot weapons that in "Defiant" are called quantum torpedoes, they emerge from the "cheek" fairings well inboard of the warp engine cowlings. Every time the pulse phasers fire, they come from the warp engine cowlings, where there are two obvious gunbarrels per cowling that serve as the emitters. Additional (and probably weaker) steerable beam weapons are seen firing from the top of the hull at the center of the top circle in "Paradise Lost" and from the round hole in the navigational deflector in a couple of episodes - both plausible as such - and are consistently the same amber "Federation phaser" color.
Also, all "guest" Starfleet vessels fire amber beams and reddish torpedoes in DS9 and VOY as well as in the newer movies, although the torpedo color really shifts to rather yellowish in VOY.
Against this level of consistency, we might accept that we see the ship fire a regular photon torpedo exactly once. In "Paradise Lost", when the Defiant makes the first run against the Lakota, the last shots exchanged are a ventral phaser beam from the Lakota at the butt of the Defiant and a single retribution from the aft torpedo tube of the Defiant. That single torpedo is red in color, like the standard photon torpedoes elsewhere in TNG era Starfleet.
We never see the Defiant fire torpedoes of other color aft, even though dialogue specifies that aft torpedoes are being fired off camera in a couple of eps ("Way of the Warrior", "The Die is Cast"). And we never see the ship fire torpedoes other than quantums forward, from locations other than the cheek fairings. So we could well argue that the ship has forward launchers designed for q-torps and a single aft tube designed for standard torps, and that the ammunition between the two is not interchangeable.
Timo Saloniemi