Eve is not real life, where wet navy ships carry a variety of weapons to deal with long range, medium range and short range threats.
The entire point behind ship to ship combat in Eve is to destroy the hostile ship in as short a time as possible, for that, you have to bring all your weapons to bear for the maximum possible time... and just as importantly, do it in as simple a manner as possible.
I have seen a lot of people who state that mixing guns, then mixing and matching ammo types to equalise ranges is the way to go, sounds great in theory, and if Eve were the real world, it would work. Unfortunately, Eve is not the real world, and often what sounds good in theory is simply impractical.
The fact is, you are creating two problems for yourself. The first, is you only have a short engagement window in which you can bring all your guns to bear on your target and inflict maximum DPS, the second is you are adding a whole new level of complexity when it comes to micromanaging your ship in combat.
What do I mean by that? Well, instead of keeping aware of the range of one weapon with all its ammo types, yu now have to be aware of the differing ranges of ALL your weapons with their ammo types... and you have a constant juggling match to ensure all your guns have the right ammo to inflict damage at the range your target is at. Add that to the fact you are likely managing your cap, you are pulsing tank to keep yourself going, you may well be pulsing speed mods to maintain your range, you are manually flying to increase or decrease transversal depending on your needs... having to switch ammo for half your guns half way through a fight because your target is too close takes your attention away from what you should really be doing, and thats a good way to lose your ship.
Eve combat simply doesnt work like that, Eve combat is not about micromanaging multiple weapons to hit a target, the mantra of Eve combat is really very, very simple.
In short, you pick your weapon, say Blasters, and you fit your ship to dictate the range, you fit your ship in a manner that maximises your chances of getting into range... and STAYING in range. With blaster ships that means MWD, scram, webs, keep your target in your guns tracking arc and make sure he stays there.
Experience has taught many of us, that mixing guns (or missiles) simply does not work. Thats not theorycrafting, we are not saying that because we think we are right, or it makes fitting look tidier. We are saying it because experience has taught us that this is correct.
In Eve you pick the range you want to work at, you pick the weapons that work at that range... then you fit your ship in such a manner that it is YOU who dictates range... not your target. In the Rock/paper/scissors world of Eve there is no one fit that can deal with all threats, its simply impossible... you avoid what you cant defeat, and you engage what you can defeat...
Only a muppet engages a Pilgrim in a blasterboat... but only a stupid pilgrim pilot engages a Drake (or a very confident one with a very expensive fit... but thats another matter)