It is very likely that between now and October there will be a schedule change, due to normal seasonal schedule changes plus changes due to the AA/US merger. Watch your reservation and if there is a schedule change, call and ask if you can just drop the MCO leg, as it no longer fits your needs.

Is this a roundtrip flight or a one way? If it's a one way you don't even need to do that. When you exit customs in Miami, just keep your bags and walk to the rental car center. No one will stop you. If it's a roundtrip, if you fail to fly the MIA-MCO leg your return flight will be cancelled.