The Faerie Dragon bestiary entry is unusual among monsters eligible to be Improved Familiars. Notably, it’s unusual because unlike all of the other available options, the Faerie Dragon has a list of Spells in addition to it’s Spell-Like Ability.
Specifically, the Faerie Dragon is explicitly called out as casting spells as a 3rd-level sorcerer.
To me, this implies that the Faerie Dragon’s list of spells known is capable of being determined at the time of ‘creation’, and isn’t fixed across all creatures of the type. Thus, if I wanted my Faerie Dragon Familiar to know Color Spray instead of Grease, or Detect Magic instead of Open/Close, that ought to be possible.
However, I fear that my interpretation might be more permissive than is intended, and that the spell list ought to be fixed to those listed in the bestiary.
Either way, it seems very clear that this spell casting does not scale with hit dice/character level as most SLA’s do. That said, I also find myself wondering how this affects the Faerie Dragon’s access to magic items. Can it use Wands and Scrolls as a 3rd level sorcerer (i.e. without a UMD check if the spell is on the proper list)? Could it use a Page of Spell Knowledge to learn an additional spell?