There's two things to pay attention to, the arc question is one. They both have different costs. Drones have a limited quantity, but are power free. Photons are unlimited, but have a (quite heavy) power cost.
So if you were to convert (in an extreme example) a K-D6D to a Photon Chucker. You'd be replacing six Type B racks with six Photons. Not even taking into consideration their firing arcs for a second, you would completely smash the power curve of the D6D by adding the 12 points of power hungry photons. I don't recall off hand just how much power the D6D has (but I want to say its 26) Obviously Overloading them all at the same time is out of the question. To retain the same characteristics of the D6D with the new armament, you'd need to add power to the ship to compensate. How much power you add would be to your tastes, and what you're trying to accomplish. I do remember a Chris Jones Mod for EAW that incorporated a lot of TNG designs, these ships had so much power to burn, you had every system on, and to max, and you still were cruising at 25-30. It was kinda fun, but also at the same time, kinda boring. It was neat to fly by and be able to use everything just because you COULD, but at the same time, you didn't have to make the interesting choices that make SFC the game that it is. (Do I divert all power to phasers, or do I try to load a photon? cuz I don't have enough to do both)