Suppose there's a pit fiend with 19 negative levels. The text on negative levels says:
For each negative level a creature has, it takes a cumulative –1 penalty on all ability checks, attack rolls, combat maneuver checks, Combat Maneuver Defense, saving throws, and skill checks. In addition, the creature reduces its current and total hit points by 5 for each negative level it possesses. The creature is also treated as one level lower for the purpose of level-dependent variables (such as spellcasting) for each negative level possessed. Spellcasters do not lose any prepared spells or slots as a result of negative levels.
But what about, say, a monster with a lower caster level (18 for pit fiends) than negative levels?
Same as if you have a character with a one level caster dip who gets two negative levels: negative caster level.
Do you have a problem with that?
Damnit, I knew I forgot something.
I mean, will the monster be able to use it's spell-like abilities?
I'd rather not sully my question with a bump, but it's still unanswered.
Don't think so, no.
I agree with GTW. If it worked with any it should work with all, and too many spells depend on caster level in ways that don't make sense if you plug in negative numbers ("duration 1 min/level," "target 1 creature/level").