Can I use a 30 amp disconnect for a 20 amp circuit?
For the disconnecting means at the equipment, yes

A 30A "air conditioner disconnect" box will work for this job, since it a sufficient current rating for the circuit and switches all ungrounded conductors.  (One theoretically could use a 20A two-pole/DPST lightswitch for this, even, but that's likely to be more expensive than a 30A A/C disconnect box.)