If you have any #14 wire in the circuit, you must put a 15 A breaker on it; it cannot be a 20 A breaker even though there is #12 wire from the breaker to the first switch. The difference in cost between #14 and #12 is small, so don't try to save a buck here - it won't work and can present a serious fire hazard. Mixing wires in a circuit can also confuse future homeowners, as they see #12 wire coming into the breaker and think they can slap a 20 A breaker on it.
It is common (or at least, not uncommon) to have both (all) outlets under the sink switched. Garbage disposals obviously need one, but having a switch on the dishwasher is also nice for child-proofing it. If you were only planning to switch one of the outlets, add another switch into your plans.