Although there is no check for it in the code, all settings below 16 won't work because there will be an integer overflow in the calculated numbers. An unpatched SoftwareSerial doesn't work below 300 baud, so you don't have software support on Arduino for 1 baud serial communication.
Baud rates that low are no problem to be implemented in a simple sketch, though. Even with a simple loop() and checking for the millis() value you'd get enough precision to communicate reliably at such low rates. What device are you communicating with? Humans?