No ship can do everything in EVE, just as no module is right for every situation. It's important to remember that you shouldn't try to do everything, or you'll just end up a Jack of all trades and a master of none.
This will be a short post, because I don't think much needs to be said on the topic. In fact, I'm constantly amazed that so many pilots seem to have never been taken aside and explained a few simple rules for fitting their ship.
First off, we have the offensive fittings of our ship. While it might sound good in theory you don't really want to have Missiles, Autocannons, and Lasers!!! Different gun types will often have different optimal ranges, they can't be grouped, and the logistics involved in all the different ammo types is just a headache waiting to happen. There are, however, a couple of examples of ship fittings where mixed guns can be warranted. These include anytime that you have more high slots than Turret or Missile hard points. The Rifter with a Rocket launcher in its fourth high slot is a perfect example.
On a larger hull, fitting smaller modules can solve a couple of problems for you. Either allowing you to still track smaller ships that have gotten under your larger guns or for using up some remaining high slots when your PG/CPU isn't enough to get in another of the larger modules. A ship thats primary purpose is Energy Neutralizing can also find plenty of utility in keeping Small Neuts around to keep a ship drained once the larger modules have done the job of getting them drained.
As to your ships defenses, Armor or Shields, pick one. Now I know you may have thought, "If one is good then two must be better!", but you'd be wrong. You'd also be well served to decide whether you want to actively or passively tank, but I think there's room to blur that line a bit. The reasons for not mixing and matching is twofold. First off, the modules themselves will take up different slots, thus making certain ships more suited to one or the other due to module layout or ship bonuses. As a rule, low slots are for armor tanking while mid slots are for shields. The other reason being that the skills for the two types of tanking are completely different and focusing on one over the other will benefit you faster than spreading yourself too thin.
Now, I'm sure many of you may have already known all this. After all, my blog is read by the truly elite of EVE Pilots, right? Right?!? OK, well then what prompts me to write all this? A recent fight with a Scythe that left me a bit confused when I looked at the killmail.
[Scythe, Wait, What?]
Small I-a Polarized Armor Regenerator
Overdrive Injector System I
Damage Control I
1MN MicroWarpdrive I
Medium Supplemental Barrier Emitter I
Small Hull Repairer I
'Malkuth' Standard Missile Launcher I, Piranha Light Missile
Regulated Light Ion Phase Cannon I, Antimatter Charge S
Medium Beam Laser I, Multifrequency S
EP-S Gaussian I Excavation Pulse
Ah, yes the deadly Missile/Blaster/Laser Hull/Armor/Speed/Shield tanked Scythe is feared all across the lowsec space lanes. Of course, this pilot was new...but not that new. I think he was created maybe two weeks after I first started playing, and was several months old by the time I caught him flying this monstrosity. I guess I was lucky that someone was there to take me aside and explain ship fittings when I first started, clearly this pilot wasn't that lucky.