What is the caster level of a spell-like ability?
I thought it always was equal to hit dice, but then happened to notice the Pixie whose CL is high as its DH.
When increasing the HD of a monster, the CL of any spell-like abilities should go up, right? What about when adding class levels to a monster?
Neither monster creation or monster advancement cover caster level of SLAs. It would appear that the caster level of SLA's does not automatically increase by adding either class levels or racial HD, unless the SLA tells you it does (like most PC race SLAs do).
Monster SLAs are mostly fixed, but the GM can change those same way he can any other stat, just bear in mind how that will affect CR.