Ruud air handlers

Question

Im installing two ruud achiever airhandlers, one for a 2.5ton unit and the other for a 3ton unit, for the airhandlers I already have #6 wire run to the location of the unit can I use #6 to a disconnect and and then from the disconnect to the airhandler can I use #12 wire. Now for the breaker... Can I use 40amp in the main panel and 15amp in the disconnect?

Thank You, Hank.