We're finishing our basement and doing the electrical ourselves. We have a bathroom that we ran 20 amps to, 12 gauge wire, but we only have a few lights in there. The lights definitely won't be using any more than one amp, so I'm wondering, can I wire in a 15 amp switch for the lights?
Yes. As long as the switch is rated for the amperage of the load served and the voltage of the circuit. Your instance is an example of this (as long as the switches are rated for the voltage).
Here is the applicable 2011 NEC code reference:
404.14 Snap switches shall be used within their ratings and as indicated in 404.14(A) through (F).
(A) Alternating-Current General-Use Snap Switch. A form of general-use snap switch suitable only for use on ac circuits for controlling the following:
(1) Resistive and inductive loads not exceeding the ampere rating of the switch at the voltage involved