Will multiple C Models work separately?
Yes they will work without any interference.
Can I add an additional Transmitter button or Receiver to my current doorbell?
Yes, depending on the model type, most of our doorbell parts are expandable and you are able to add on extra receivers or transmitter button. Expandable models include the STARPOINT series here: http://www.amazon.com/gp/dp/B00LFATHKS/
It appears one of my SadoTech Model C door Transmitters has gone bad, can I buy just a Transmitter?
Yes, depending on the model type, most of our doorbell parts are replaceable and can purchased separately.