I read all over the net that a problem with mages as followers are that they use upp all their Fatigue at the beginning of the battle with spell-attacks and then are useless for the rest of the battle.

Well, in my game they never use attack-spells. They always fight with weapons. I have tried to take away their weapons and then they fight with their bare hands instead. But they never uses any magic exept healing.
Perriman Smythe sometimes summon a big cat but thats really rare.

They are useless fighters so I don't get it. Z'an Al'urin keep hurting herself because she fights with her hand or her magic staff instead of using magic.

So, am I missing something? Is it a bug? Or is it like that for all of you also?

Another problem is the followers with guns. They almost always run up to the enemy before shooting. They have ranged weapons! They don't HAVE to stand right infront of the enemy.

Yeah the ai is pretty bad, especially with casters.

Firstly, it's better for casters in real time. They will cast more spells in real time than in turn based.

Sometimes they cast off all their spells at once, sometimes they only do a little and never cast again. Usually, they just don't cast. In turn based you're lucky to get 2 spells from a caster ally each fight.

The exception is healing spells, which trigger at certain hp levels based on what ai scheme the follower uses.

Raising follower mana/fatigue does help with the casting though.

This is purely speculation, but with the gunslinger followers I think the AI is computing the to-hit chance based on distance and lighting penalties, and deciding that a point-blank shot is the way to go. Like ewmarch says, it's a problem with the implementation of the AI.

For missile-weapons, the AI may also be dealing with getting a clear shot so as to avoid hitting allies.

Bottom line, since you can't change the AI;
unless you're recruiting NPCs for RP and story reasons, favour melee fighters and healers over ranged fighters or mages.

There are several very good melee fighters NPCs in the game (both half-ogres, the half-orc, Magnus, Torian Kel, etc...), they should be enough to fill your party without having to use the useless mages.