I think Faerie Dragons or other creature cannot use spells to attack might be a bug.
Just means it needs a button for them, or a new menu to choose spells.
There are two buttons in the creature menu, if in the battle,this button can be shown here, click the button and the creature can cast a spell.
Left one for Faerie Dragons and right one for Magogs and Liches, if any one designs another kind of this creature,it can also be used.
(Attack Like a Spell also can be used in keyboard, click F for choosing a range)
Faerie Dragons are random caster, so just click the button and the spell will be randomly select.
However, if some creatures can choose the spells itself,it will be designed as:
Like H4, the creature will have own spell point, like faerie dragons have 5 points, if they use the Chain Lightning, it will consume 2 points, and 3 remains. The little spell icon can be found in SpellInt.def.