You should really concentrate on one form of tanking, and then use the rest of your spare slots to fit a damage control (by far the best single module in the whole game for all types of tanking!), weapons, weapon modifiers, electronic warfare and tackle modules. If you try to fit two forms of tank, or more, you compromise your first tanking method.

Active tanking both armour and shield is obviously wrong as you run out of capacitor really fast so you fit a tank that only really does half a job before it runs out of energy and the other half is incapable of doing anything.

Passive tanking is not so obviously wrong, but is still not good for you. A ship with massive buffer tank is slower because of the armour buffer and so is easier to hit. It is also easier to hit because the shield buffer makes it a bigger target in game mechanic terms (ship signature). The ship has a very large number of total hitpoints, but that number will go down much quicker than a ship with just armour or shield buffer tank because you have made the ship much easier to hit.

You must also consider what else you want your ship to do. Low slots are good for damage, shield and speed modifiers in a shield tanked ship, or armour modifiers in an armour tanked ship. Mid-slots are useful for a MWD/AB and tackle/ewar/weapon modifiers on an armour tanked ship, and shield modifiers on a shield tanked ship. If you fit both armour and shield tanking modules (even those modules that do not affect speed/signature, such as resitance modifiers) you are using slots that you could put to better use to end the battle quicker. End it quick and your tank will not be stressed too much; take too long and your tank may fail before the other guy's, no matter how big it is.

Speed tanking is not good to mix with any other form of tanking, either. Armour tanking reduces your speed and uses up your low slots, and you can fit a much better shield tank if you use low slots for shield recharging and damage controls instead of speed modules. Also, the point in speed tanking is to avoid being hit as much as possible, but both armour and shield tanks make you easier to hit or use up your valuable capacitor energy.

Hull tanking is for heroes, so should never be combined with shield, speed or armour tanks just in case someone calls you a coward. Even if you did mix tanks with hull tanking, you use up low slots in speed and armour tanks, and mid slots for a shield tank, so you will be unable to "hero up" your hull properly or fit enough hull repairers.

All joking aside, the game is setup so ships can do three things: fly, tank and destroy. Concentrate on only one form of each with your ship to maximise your potential survivability and success rates.