Never mix ranges.
I say ranges because it includes mixing different weapon types, and charges (crystals, ammo) that has different ranges. Why not? Simple: your enemy will only ever be in one place. He may move from place to place, but will never occupy multiple places at once.
Here's a nifty graph to help (click to view a larger version). Why did I choose a Coercer and a Rifter to illustrate? Because of this kill that I got. The graph above may not be to scale, since the Coercer's damage sucked so much that it did about 300 damage with 8 guns in the same time that it took me to deal 1888 damage with 3 guns. Now, in response to the graph, some might say
But what if the Rifter is smart and moves out of your focused range, and you would be doing more DPS with a multi-range fit?Then you'd be wasting the DPS in the long-range guns/ammo you have fitted. The meager DPS that you would be dealing would only serve to keep you aggressed and unable to flee a fight that you are clearly not prepared to fight.
The Coercer could have done that. He was autopiloting (through 0.0 security space!) which means his autopilot took him to 20 km off the gate, then flew to the gate. I wasn't dealing enough damage to pop him before he got to the gate, and my Thrasher friend wouldn't have gotten there in time to finish him off. But he engaged me away. Pro tip: frigates are small, but that doesn't mean they pop in one hit to any bigger ship.
If this entry really disappointed you, you may want to train your drone skills and fly Gallente ships. Drones are range-independent to a large degree. However, they can get shot down. Properly used guns deal just as much damage, and can't.