Can I run one pendant light off garbage disposal switch?
You can't connect up any kitchen lighting to any of the dedicated small appliance circuits. And any receptacle near a counter automatically becomes a dedicated small appliance circuit.
The solutions include:

New circuit - kind of crazy given the low power needed for modern LED lighting, plus the work involved.
Connect to existing lighting circuit - that is the most logical, but not always the easiest
Connect to a circuit in the kitchen that is not connected to any small appliance receptacles, like the garbage disposal or dishwasher. That's your idea, and it should work.

If I remember correctly, you can't normally have receptacles on a circuit that has 50% or more capacity used by hardwired loads. For example, if a circuit has a 10A garbage disposal or dishwasher then you can't put a regular receptacle on the same circuit. But you should be able to add more hardwired loads (like lighting) as long as the total does not exceed 80% of the capacity of the circuit (it gets a little more complicated than that, as only continuous loads (like lighting) need to be derated but not temporary loads (like a garbage disposal that runs for a few seconds at a time), but better to stay well within the capacity of the circuit).
There is another reason, aside from code compliance, to not put lighting on small appliance circuits: GFCI. The small appliance circuits require GFCI protection. Lighting generally does not, and in fact it is generally better to not have lighting GFCI protected because if it is protected and a ground fault occurs on the circuit, the lights go out! Leaving you in the dark next to a broken food processor or whatever.